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The aim of the present work was to estimate the maintenance requirement for nitrogen of guinea-pigs and to examine the effect of different contents of crude protein and energy in diets on the development of body weight as well as on the changes of the chemical composition of the animal body of female guinea-pigs.On the basis of eight female guinea-pigs aged between 35 and 39 days the maintenance requirement for nitrogen wasestimated. The average faecal N totaled to 109,72 mgN/kg W 0,75.d whereas 139,45 mg N/kg W 0,75.d were measuredfor the urinary N. It was observed that the faecal N is significantly dependent (positive correlation) on the intakeof dry food substance. Including the known amount of dermalnitrogen loss (20 mg N/kg W 0,75.d) the nitrogen maintenancerequirement of guinea-pigs in the 5th and 6th week of livingamounted to 269,17 mg N/kg W 0,75.d.A growth experiment over a period of 42 days was performed with 64 female guinea-pigs aged 15 days of the strain Iv: Dunkin-Hartley-pirbright with an average initial body weight of 161,5 g. This experiment was to investigate the influence of purified diets differing in their content of crude protein and energy on the development of the body weight and on the changes of the chemical composition of the animal body. The diets contained the following contents of crude protein and gross energy in one kilogramme of air dried substance: diet 1 (143,5 g crude protein; 14,45 MJ gross energy), diet II (183 g crude protein; 14,53 MJ gross energy), diet III (133 g crude protein; 18,17 MJ gross energy), diet IV (171,5 g crude protein; 17,01 MJ gross energy).In addition to the growth experiment, an another experiment was performed to determine the apparent digestibility of crude protein, crude fat and energy during the 3rd and 4th week. The digestibility of crude protein ranged from 90 to 92 per cent whereas the digestibility of crude fat was between 77 and 91 per cent and the digestibility of energy ranged from 81 to 86 per cent. The different digestibility of energy resulted in the following energy-proteinrelationship (kJ digestible energy per g crude protein) of the diets: diet l (86,89), diet II (63,77), diet III (112,63), diet IV (84,84).At the end of the growth experiment the following results were ascertained:- The cumulated increase of body weight of animals of all feeding groups showed satisfying results.- The cumulated food intake depended an the energy-proteinrelationship; the wider the relationship, the fewer the intake of food (saved statisticallv).- The food expenditure was decreased by a wide energyprotein-relationship and increased by a close one (saved statistically).- The PER was influenced by the intake of crude proteinand the energy-protein-relationship. More favourable results were measured under the condition of wider energy-proteinrelationship and lower crude protein intake (saved statistically).- The analysis of the whole animal body performed at the end of 2nd, 4th and 6th week of the experiment demonstrated decreasing crude water contents and increasing contents of dry substance, crude protein and crude fat of the animal body. The values did not differ significantly within the groups. Nevertheless the crude fat portion in one kilogramme dry substance of the group III was significantly higher than that of groups II and IV. At the end of the experiment the crude protein content of the retention ranged from 18,62 to 19,76 per cent. As measured at the end of the experiment the retention of crude protein, crude fat, protein energy,fat energy and the entire energy in he animal body did not show any significant differences between the groups. A significant positive correlation between the retention of crude protein and crude protein intake and intake of digestible energy was observed. - Both "apparent NPU" and NPUop correlated negatively with the crude protein intake and they were positively influenced by the wideness of the energy-protein-relationship. - A dependence of relative weight increase of the kidneys an protein intake was observed.According to the present results a energy-proteinrelationship of 85 to 90 kJ digestible energy per g crude protein in purified diets with a crude protein content of 14 to 18 per cent and a content of digestible energy of 12 to 16 MJ in one kilogramme diet is suggested for growing female guinea-pigs.