Litter sizes in inbred strains of rats (Rattus norvegicus).

The average litter sizes and reproductive performance of twenty-five inbred strains of rats, maintained in a conventional colony in Pittsburgh, and twenty-six inbred strains, maintained in a barrier facility at the National Institutes of Health, were calculated from data collected for periods of 3 and 10 years, respectively. There were considerable variations in both parameters in each of the two colonies studied. In addition, there were quite wide variations in male to female ratios among inbred strains (0.80 to 1.42).