To minimize the ecological impact of oil at sea, dispersants are used to spread the spill across a large amount of water and thus make hydrocarbon levels low enough to be sublethal. Such an objective demands two correlative conditions: (1) that an oil-in-water emulsion can be obtained by adding the dispersant and (2) that rapid dilution conditions are provided by a downward motion of oil droplets and wind-induced velocity shear between surface and subsurface. In September 1981, two 6-m3 slicks in the Mediterranean Sea were treated by two concentrated dispersants. More than 1500 samples were taken at different depths during several hours following treatment and were analyzed by a differential method to measure separately oil and dispersant. Direct measures were also recorded by a continuous flow of water through a nephelometer and a spectrofluorometer. Despite apparently complete emulsification of the slick, the most efficient dispersant did not preclude some of the oil from resurfacing 6 h after treatment. The short-term fate of dispersed oil was therefore under the control of the sea conditions, which limit dilution processes. Because any laboratory test in closed vessel cannot duplicate dilution factors, a new approach is being developed to measure simultaneously the rate of dispersion and the behavior of test marine animals submitted to variable concentrations of potentially toxic substances during the flow of dispersed oil through a continuous system.