Three experiments were conducted to investigate the genotoxic effects of crude oil on mink and sea otters, In the first experiment, the effects on mink of chronic exposure to weathered Prudhoe Bay crude oil were studied, Female mink were fed a diet that included weathered crude oil for a period of 3 weeks prior to mating, during pregnancy and until weaning. Kits were exposed through lactation and by diet after weaning until 4 months of age. Kidney and liver tissues of the kits were examined using flow cytometry (FCM) and it was found that the genome size was increased in kidney samples from the experimental group compared to the control group. This effect was probably due to some type of DNA amplification and it could have been inherited from the exposed mothers or have been a somatic response to oil exposure in the pups, No evidence of clastogenic effects, as measured by the coefficient of variation (CV) of the G(1) peak, was found in kidney or liver tissue. In the second experiment, yearling female mink were exposed either by diet or externally to crude oil or bunker C fuel oil. Evidence for clastogenic damage was found in spleen tissue for the exposure groups, but not in kidney tissue. No evidence of increased genome size was observed. In the third experiment, blood was obtained from wild-caught sea otters in Prince William Sound. The sea otters represented two populations: one from western Prince William Sound that was potentially exposed to oil from the Exxon Valdez oil spill and a reference population from eastern Prince William Sound that did not receive oil from the spill. The spill had occurred 1.5 years prior to obtaining the blood samples. Although the mean CVs did not differ between the populations, the exposed population had a significantly higher variance of CV measurements and five out of 15 animals from the exposed population had CVs higher than the 95% confidence limits of the reference population, It is concluded that FCM is a sensitive indicator of the clastogenic effects of oil exposure and that haematopoietic tissues and blood are best for detecting clastogenic damage. Moreover, the observed differences in the genome size of the kidney cells mere possibly heritable effects, but this needs further investigation. Lastly, sea otters exposed to spilled oil 1.5 years earlier showed evidence of clastogenic damage in one-third of the individuals sampled.