Oil slicks in laboratory test chambers inhibited swimming behavior of rice rats, and reduced survival at low temperature. Predisposition to enter the water and swim was greatly reduced at both high (200 ml/m2 water surface) and low (20 ml/m2) concentrations of oil. Survival was significantly affected only at high concentrations. The results may be of value in predicting the impact of oil spills on the mammal community of coastal marshes.