Fathead minnow larvae (Pimephales promelas) were exposed to three individual pesticides during brief or continuous exposure in 96-hr and 28- to 30-day toxicity tests. Continuous exposure 96-hr LC50 values for chlorpyrifos, endrin, and fenvalerate were 122.2, 0.7, and 0.85 micrograms/liter, respectively. Continuous exposure chronic effect concentrations were chlorpyrifos, 2.1 micrograms/liter (increased deformities); endrin, 0.38 micrograms/liter (reduction in growth); and fenvalerate, 0.36 micrograms/liter (reduction in survival and growth). Brief exposure chronic test results indicated that fathead minnow exposure to chlorpyrifos for as few as 5 hr at a concentration similar to a continuous exposure 96-hr LC50 value resulted in increased deformities and a reduction in growth, whereas a 48-hr exposure at a concentration similar to a continuous exposure 96-hr LC50 value was required to cause a reduction in growth for endrin and a reduction in survival and growth for fenvalerate. It is suggested that although constant exposure laboratory tests are essential for hazard assessment, the relationships of exposure duration and toxicant intensity to ecotoxic effects are necessary for reliable risk assessments and implementation of water quality standards.