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The objective of this study was to investigate the effect of gestation and lactation on the metabolism of conventionally fed sows with respect to changes in body weight, backfat thickness and thorax girth.During gestation and lactation ten primiparous sows (P1) and ten multiparous sows (parity 3; P3) were monitored. Seven blood samples were taken by puncturing the vena cava cranialis and vena jugularis, respectively. In parallel with blood samples body weight, backfatthickness and thorax girth were measured. At insemination, on 110th day of gestation and at weaning body condition was scored.All animals were clinically healthy. The blood samples were examined for concentrations of urea, creatinine, free fatty acids, triglycerides, betahydroxybutyric acid, glucose and insulin.Following dependencies were discovered:• Body weight, backfat thickness and thorax girth changed significantly during the reproductive cycle.• In the present study gilts must gain 38,5 kg and mutiparous sows 63,0 kg of weight during gestation to achieve a backfat thickness of 20 mm in late gestation.• On the 85th day of gestation gilts and multiparous sows already mobilized body reserves. Despite an increased feeding level backfat stagnated between 85s" and 110d" day of gestation. In this period thorax girth of gilts was constant, whereas in the group of multiparous sows a decrease of 2,9 % was noted.• During lactation both groups lost body weight (P1 = 18 kg (9 %) vs. P3 = 17,5 (7 %). Backfat (P1 and P3 = 18,8 %) and thorax girth (P1 = 5,6 % v. P3 = 3,7 %) decreased. Gilts lost slightly more weight and thorax girth than multiparous sows (p < 0,09). • Serum concentrations of urea, creatinine, free fatty acids, triglycerides, betahydroxybutyric acid, glucose and insulin were significantly changed in the course of gestation and lactation.• In both groups concentration of urea increased in late gestation and lactation.• In the P1 group was a very significant correlation between the relative exchange of thorax girth during the period of 85th and 110th day of gestation and the concentration of urea on the 110th day of gestation (p = 0,005 ; r = 0,8). The relative exange of thorax girth during the first half of lactation correlated to the concentration of urea on the 10th day of lactation (p < 0,001 ; r = 0,89). Changes of thorax girth reflect changes in thorax muscles. The decrease of horax girth in combination with increased concentration of urea demonstrated protein catabolism.• Because of higher body weight and more muscular tissue in the P3 group creatinine concentration was always higher than in the P1 group. This differences were significant on 60`h and 110th day of gestation and on 3th and 21th day of lactation.• Because of intensive fetal growth in both groups creatinine concentration increased in late gestation. Decreased creatinine concentrations after farrowing showed its dependence on fetal muscle metabolism. Creatinine concentrations after farrowing were constant duringlactation.• In both groups the concentration of free fatty acids increased in late gestation and in the second half of lactation.• In the P1 group the concentration of free fatty acids on the 21 th day of lactation was significantly correlated to the percentage backfat loss in lactation (p = 0,05 ; r = 0,7), especially in the second half of lactation (p < 0,05 ; r = 0,8). Increased free fatty acid concentrations reflected the mobilisation of body fat. Furthermore liver and mammary cells produce free fatty acid from blood glucose.• In both groups concentrations of beta-hydroxybutyric acid increased only at the end of lactation. In this investigation increased concentrations of beta-hydroxybutyric acid caused no ketosis.• In both groups triglycerides concentration was highest on the 110"" day of gestation. Increased estradiol and prolactin concentration will increase triglycerides concentration by inhibition of lipoproteinlipases. Concentration of triglycerides decreased on the 3th day after farrowing to the level of the 85th day of gestation (p < 0,05) and was constant during lactation.• In the PI group there was a significant correlation between the concentrations of triglycerides and free fatty acids on the 21th day of lactation (p < 0,05 ; r = 0,8). Increased triglycerides concentrations could result from resynthesis of triglycerides from free fatty acids.• In both groups glucose concentration increased in late gestation. Highest concentrations were noted at the beginning of lactation. On the 10th and 21th day of lactation glucose concentration decreased. Increased glucose concentrations at the beginning of lactation could result from increased cortisol levels at farrowing. On the other hand after parturition suckling stimulates the release of oxytocin and glucagon. These hormones increase blood glucose by promoting gluconeogenesis. The decrease in blood glucose during a 3 - week lactation could reflect the utilization of glucose by the mammary gland.• The increased glucose concentrations were not reflected by increased insulin concentrations. Increased glucose concentrations with an insulin resistance in late gestation are regarded as a physiological adaptation of the maternal organism to provide more carbohydrates for the fetal organism. Concentrations of insulin increased at the beginning of lactation but dropped during the course of !action.