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In this study the preference of 104 male and female HAN:WISTAR rats of two ages was tested withMacrolon cages of different size and structure. The animals were kept in groups of four rats, samesex and age. This study was aimed to find clues about proper housing conditions of laboratoryrats.Two series of tests were carried out:1. Experiments on cage size: Three Makrolon cages of different size, connected by tubes, wereused.2. Experiments on cage enrichment: Two Macrolon cages of the same size (Typ IV), connected bytubes, were used. One cage contained litter, the other one was filled with different devices ofenrichment during five continual experiments: a) an inset for hiding made of plastic . b ) anincreased cage top, c) a large layer of litter, d) gnawing materials: straw, pieces of wood, cereals.e) a combination of all of these enrichments.In both studies the behaviour of each group was filmed using a videocamera. Locomotor activitywas monitored by light barriers and infrared-computerddectors. Periods of bright and dark lightwere analysed separately from the recorded tapes. Finally we developed ethograms to determinepreferences for the single cages and structuresResults of the experiments on cage size:90 % of the rats spent their resting time during the light period tightly pressed to one anotherin the smallest cage, Typ II. During the day single rats wall;ed through all three cages inunregularly intervals in order to explore.All rats were night active. No regular activity patterns could be stated. Sequences of activitydiffered individually and from night to night. During the activity periods in the night all cageswere used.However, there was a distinct preference for performing certain behavioural elements in therespective cage types.Cage type: II; Ground floor (cm2): 350; Dwelling time (night tapes): -; Locomotor activity: --;Behaviour elements: resting, grooming, social contact.Cage type: III; Ground floor (cm2): 830; Dwelling time (night tapes): +; Locomotor activity: ++;Behaviour elements: drinking, eating, digging, resting, social contact.Cage type: IV; Ground floor (cm2): 1800; Dwelling time (night tapes): ++; Locomotor activity: +++;Behaviour elements: playing, climbing, running, social contactTab. 56 :Summary showing the results from the analysis of the video tapes taken in the nightWith increasing group age locomotor activity decreased, playing sequences were reduced andoccured rarely whereas exploration increased. Social contacts could be registered from all animalsin each cage. Abnormal behaviour, aggressivness and physical damages could not be realized.Results of the experiments on cage enrichment:Enriched cage: Nearly 100% of the rats spent their resting time under the plastic inset. During thenight time the inset was used as a "view platform", moreover, it was used for eating, drinking andshort-time resting. The other devices of enrichment were specifically used: straw was used fornestbuilding, wood was moved and gnawed on, cereals gave an additional nutritive stimulus toemployment, litter was piled up and was also used for nestbuilding. Nightly sequences of activityoccured all over both cages.Non-enriched cage: Here eating an drinking took place as well as urination and defacation. All ofthe animals were tame and easy to handle.Conclusions:Laboratory rats need a minimum of cage size for fast locomotor activity and for structuring theirliving area. These demands are offered most easily by a Macrolon Type IV cage.A suitable cage for ats should contain various enrichment devices because rats have the needfor stucturing their cages. A cage inset for hiding offers simultaneously a second level within thecage and thus enlarges cages size.In order to fulfill not only the demands of rats, but also those of economy and science, housingof not more than four to five rats in an enriched Type IV cage seemes to be a suitablecompromise.Additional results from ethological and physiological investigations are needed in order to findimproved housing standards which will meet the physiological and ethological needs of laboratoryrats.