An exposure system and method of quantitating toxicity have been developed to provide an estimate of the effects of dispersed oil on marine organisms under a variety of exposure conditions. Results of constant concentration exposures (for hours or days) can be compared to those of diluting exposures (decreasing to zero in 8 or 24 h) on a basis of the “toxicity index.” This index is equal to the total exposure when time in hours or days is multiplied by the concentration at each hour (ppm · h or ppm · days). Tests have been conducted with shrimp (Pandalus danae), two oils (Prudhoe Bay crude and a light Arabian crude), and two dispersants. There is a seasonal pattern to the tolerance of the shrimp. Tests in the colder months (fall/winter) produce toxicity indices approximately three times higher than summer/spring values. Testing shrimp with Prudhoe Bay crude oil and a chemical dispersant during the fall/winter season, we found constant and 24-h dilution exposures produced toxicity indices of 11 (±1.1 standard error) and 10 (±0.6 standard error) ppm · days, respectively. During the fall/winter season (greatest tolerance), tests with Prudhoe Bay crude and two different chemical dispersants produced toxicity indices for P. danae of 10 (±0.6) and 12 (±1.1) ppm · days. During tests in summer, there was also little difference observed when the toxicity of the light Arabian oil was compared to that of Prudhoe Bay crude (2.3 and 3.4 ppm · days, respectively). The usefulness of our methods is that in addition to the comparisons already noted, it is possible to predict the outcome of dispersant application under varying environmental conditions.