The effect of exposure duration on the toxicity of a forest insecticide (carbaryl) was assessed under environmentally realistic exposure regimes against two stream invertebrates indigenous to the United States Pacific Northwest, Calineuria californica (Plecoptera: Perlidae) and Cinygma sp. (Ephemeroptera: Heptageniidae). Laboratory bioassays were conducted to evaluate the relationship between pulsed exposures of 15, 30, and 60 min and toxicity for a range of chemical concentrations (10.2-1,730 microg/L). For Cinygma sp., the 50% lethal concentration (LC50) values were calculated as 848 microg/L (15 min), 220 microg/L (30 min), and 165 microg/L (60 min). The C. californica consistently had lower mortality at a given concentration compared with Cinygma sp. Fifteen- and 30-min exposures did not elicit 50% mortality with C. californica, and it had a 60-min LC50 of 1,139 microg/L. Time to 50% mortality over 96 h after a 15-, 30-, or 60-min exposure, with the rest of the test period in freshwater (PLT50), was a function of exposure duration and concentration. Analysis of symptomology throughout the test period for C. californica gave evidence of recovery from the knockdown and moribund states, but this was not the case for Cinygma sp. The pulse duration resulting in 50% mortality was calculated as 43 min for Cinygma sp. exposed at 204 microg/L and 16 min at 408 microg/L. A three-dimensional probit plane model [Y = -10.86 + 4.83(ln C) + 3.0(ln T)], where Y is probit mortality, C is concentration in microg/L and T is time in hours, was used to explain the interaction between concentration (microg/L) and duration of exposure (hours) for Cinygma sp.