The acute effects of crude oil on glucose uptake rates by marine microorganisms were studied in 215 water and 162 sediment samples collected from both arctic and subarctic marine waters. The mean percentage reduction of glucose uptake rates ranged from 37 to 58 in the water samples exposed to crude oil and from 14 to 36 in the sediment samples. Substrate uptake kinetic studies indicated that the observed reductions by microbial populations exposed to crude oil were caused by metabolic inhibition. The effect of crude oil was less in sediments than in the water samples, with the difference being significant at the P < 0.0002 level. With the exception of one sediment study, all of the differences observed in the uptake rates between treated and nontreated samples were statistically significant. A high degree of variability was observed in the degree which glucose and glutamate uptake rates were altered in water samples exposed to crude oil. In some cases, uptake rates were greater in the samples exposed to crude oil. Data on samples collected in Cook Inlet suggested that areas where pelagic microorganisms are most probably chronically exposed to crude oil are also the areas where the effects of crude oil on glucose uptake are the lowest. Two studies indicated that after pelagic populations are exposed to crude oil for several days, the heterotrophic population adjusts to the presence of crude oil.