Sixteen inbred strains of mice and seven inbred strains of rats had been introduced from foreign countries since 1985, and were maintained in plastic film isolators in our institute. All of these strains, as foundation colonies, were kept according to a modified parallel line system with full brother-sister mating, accompanied with complete pedigree records including breeding record cards, pedigree charts and pedigree books. Up to now, except three mice and one rat strains which were extinct before 4 generations, all other strains survived to 11-22 generations. The environmental conditions for the animals were carefully controlled, including temperature, humidity, microbiological status and nutrition. The reproductive performance and body weight growth of these strains were observed, and data obtained here were different from previous reports to some extent. The results showed that many environmental factors could affect reproduction and development of the inbred strains. These effects must be taken into cosideration when the animals are used in long ter n experiments involved in animal reproduction and growth.