The influence of environmental contaminants on the muskrat population inhabiting the contaminated lower region of the Elizabeth River was studied through an analysis of contaminant burdens, physiological characteristics, and population dynamics in comparison to those of muskrat populations from a less contaminated region of the Elizabeth River (upper region) and a nearby uncontaminated river (Nansemond River). A total of 76 muskrats was collected for contaminant analysis during December 1986 - January 1987 and December 1987 - February 1988. Twenty-two of 35 carcasses analyzed for polynuclear aromatic hydrocarbons (PAHs) had detectable levels of from 1 to 6 PAH compounds. Only three muskrats from the lower region and one from the upper region of the Elizabeth River had PAH concentrations greater than 0.03 ppm dry wt (lower limit of detection). Liver DNA adduct levels were not significantly different between muskrats collected from the lower region of the Elizabeth River and muskrats collected from the Nansemond River. However, liver microsomal enzyme activity was greater in lower region Elizabeth River muskrats than in upper region Elizabeth River or Nansemond River muskrats, as indicated by significantly reduced pentobarbital sleeping times. The mean concentration of 14 PAH compounds detected in surface sediments from the lower region of the Elizabeth River (N = 10) was significantly greater than the mean concentration detected in surface sediments collected from the upper region of the Elizabeth River (N = 5) or Nansemond River (N = 5). Of 22 organochlorine compounds analyzed in 35 muskrat carcasses, dieldrin was detected in one carcass (0.25 ppm) from the lower region of the Elizabeth River, polychlorinated biphenyls were detected in two carcasses (0.66 ppm and 0.45 ppm) from the upper region of the Elizabeth River, and p,p′-DDE was detected in two carcasses (0.03 ppm each) from the upper region of the Elizabeth River and one carcass (0.03 ppm) from the Nansemond River. p,p′-DDE was detected in 5 of 10 sediment samples from the lower region and 2 of 5 sediment samples from the upper region of the Elizabeth River. p,p′-DDD was detected in 3 of 10 sediment samples from the lower region of the Elizabeth River. Twenty-seven of 33 metals analyzed were detected in muskrat kidneys and 9 of these were significantly different among the three study regions. Mean aluminum (13.19 ppm), cadmium (3.08 ppm), copper (12.85 ppm), nickel (0.50 ppm), and zinc (88.38 ppm) concentrations were greatest in lower region Elizabeth River muskrat kidneys. Mean cadmium (1.07 ppm), chromium (43.4 ppm), lead (104 ppm), tungsten (38.1 ppm), and mercury (0.50 ppm) concentrations were significantly greater in lower Elizabeth River sediment samples. Density estimates based on shore length for the lower and upper regions of the Elizabeth River were 0.86 muskrats/IOO m of shore and 1.1 muskrats/lOO m of shore, respectively in 1987. Seventy-five female muskrats had a total of 637 placental scars (x̅ = 8.49) ranging from 1 - 20. The number of placental scars per female did not differ significantly among regions. Twelve pregnant muskrats had a total of 54 fetuses (x̅ = 4.5, range = 3 - 6). Average number of litters per year was estimated to be 1.89 with births occurring primarily from April - May and in September. Results indicated that the environmental contaminants found in the lower region of the Elizabeth River have minimal influence on the muskrats from this region. Body and spleen weights were reduced but reproduction was not affected, and the muskrat density in this region appears to be stable and similar to the density in a less contaminated area. Immunological function may be depressed. / Ph. D.