Prediction of pulsed or intermittent exposure effects on populations is emerging as an important issue in ecotoxicology. However, the underlying theory of the dose-response models has not been tested rigorously enough to provide a true understanding for predicting the effects of pulsed exposures. Since its introduction, the individual effective dose (IED) theory has remained the dominant explanation in the literature. The present study tested whether the IED theory is the dominant explanation for the probit model (or similar models) for both copper sulfate (CuSO4) and sodium pentachlorophenol (NaPCP). Three groups of amphipods (Hyalella azteca) were first exposed to lethal, sublethal, and zero concentrations of each toxicant. After recovery, all survivors were exposed to lethal concentrations, and their mortalities during a second exposure were compared. Under the specific concentrations and durations used, stochastic processes were dominant for CuSO4. Both stochasticity and IED appeared to be relevant for NaPCP, but stochasticity dominated the dynamics. Current ecotoxicology tests should include assessment of the underlying dynamics as one of the first steps of studying pulsed or intermittent exposure effects.