To determine changes in pregnancy rates, a collection of Norway rats (Rattus norvegicus) was obtained from poultry warehouses in Baltimore, Md., during a period of 14 mos., which were divided into 8 seasons. Data on size and reproductive condition were obtained at autopsy. The sample of 597 females contained a high proportion of small rats in the spring and fall and also at the beginning of collection. Of the 1318 rats captured, 54.4% were males. The analysis of the prevalence of pregnancy showed no significant variation among seasons for the large rats and an avg. of 40.3% pregnant. The prevalence for small rats was high in the spring and fall and low in the summer and winter and averaged 10.8%. Annual litter rates varied from 8.20 for large rats to 2.19 for small rats. The mean no. of embryos in 105 pregnant rats was 9.28 [plus or minus] 2.28. Seasonal change in prevalence of lactation was an increase to a steady level for spring and summer and a sharp decline in the fall. There is no seasonal closure of the vagina nor regression of the uterus. To determine relative ages of rats, the size at which 50% of the rats have the 3d molar erupted (64 g.) or the skull ossified (98 g.) or the vagina perforated (102 g.) or follicles ovulated (153 g.) or placental scars present (318 g.) may be calculated.