SummaryHousing systems for laboratory animals have been developed over a long tim e. Microenvironmental systems such as positive, individually ventilat ed caging systems and forcedair-venti lat ed systems are increasingly used by many researchers to reduce cross contamination between cages. T here have been many investigat ions of the impact of these systems on the health of animals, the light intensity, the relative humidit y and temperature of cages, the concentration of ammonia and CO 2 , and other factors in the cages. T he aim of the present study was to compare the effects of different rack systems and to understand the in¯uence of environmental enrichment on the breeding performance of mice. Sixty DBA=2 breeding pairs were used for this experiment. Animals were kept in three rack systems: a ventilat ed cabinet, a normal open rack and an individually ventilated cage rack (IVC rack) with enriched or non-enriched type II elongated Makrolon cages. Reproduction performance was recorded from 10 to 40 weeks of age. In all three rack systems there was a similar breeding index (pups/dam/week) in non-enriched groups during the long-term breeding period, but the coef®cients of variation in the IVC rack were higher for most parameters. T his type of enrichment seems to lead to a decrease in the number of pups born, especially in the IVC group. However, there was no signi®cant difference in breeding index (young weaned/female/ week).