Federal guidelines provide recommendations regarding the minimum of floor space that should be allotted for breeding laboratory rodents. Mouse mating systems used by a variety of institutions frequently deviate from these recommendations. Regulatory agencies suggest that deviations from established guidelines should be assessed on an institutional basis and recommend periodic review by the local IACUC. Mouse breeding data, collected in a laboratory animal management software program at a single institution, were retrospectively analyzed to determine the effects of 2 breeding schemes on reproductive performance. Data were analyzed over a 20-mo period from all inbred strains of mice breeding in the vivarium. Variables included total number of pups per litter, pups per female, and litters per female. Data indicated that the numbers of pups and litters per dam do not differ between trio-bred (one male, 2 female) and pair-bred (one male, one female) mice. However, more pups were weaned per litter in trio-bred (mean, 5.8 pups) when-compared with pair-bred (mean, 4.7 pups) mice. These results suggest that allocating less than the recommended amount of floor space is not detrimental to the overall breeding success of the strains of mice examined.