Muskrats, Ondatra zibethicus, (360) were collected from river and creek study areas in East Tennessee from July, 1972 to June, 1973. Data were recorded on reproductive parameters. Maximum values for testis length and width occurred in August, whereas maxima for testis volume and weight were in May and August, respectively. Evidence indicated that sperm were present in the adult males year-round. Maturation of follicles began in January in adult females; mature follicles were present in late February and March, and the first corpora lutea of pregnancy were found in April. Maximum values for mean ovarian weights for pregnant and non-pregnant river adults were achieved in July. Ovarian lengths were maximum in April (non-pregnant) and August (pregnant). Average litter size was 5.38 and the average number oflitters per year per female was 2.3. Fetal implantations were found until August, and follicular activity ceased by October. The peak months of prevalence of pregnancy were April, May, and July when 50 percent of the females were pregnant by gross examination. It was estimated that 36 to 50 percent of the sample was in the preimplantation stage of pregnancy and thus not accounted for as pregnant animals. Annual productivity in the river females was not significantly greater (5% probability level) than in the creek females and thus did not reflect nutritional differences that apparently exist between the two areas. Records from the present study indicate litter sizes are intennediate between studies from more southern and northern latitudes.
