Fathead minnows, Pimephales promelas, were exposed to endrin or DDT under static and dynamic conditions to evaluate whether the particular condition would appreciably change the apparent insecticide toxicity. Comparative 48- and 96-hour endrin exposure indicated a slightly higher LC50value during static tests as compared with dynamic tests. This may be due, in part, to the maintenance of endrin concentrations in the latter. Comparative 48-hour DDT bioassays indicated the static condition to be much more toxic than the dynamic. Because toxicity was higher in the static system, where DDT levels were decreasing rapidly, it may be reasonable to assume that decreasing oxygen concentrations and/or accumulating fish metabolites (ammonia, CO2, and others) may have enhanced the apparent toxicity of DDT during static exposure.