The impact of hexazinone and metsulfuron methyl herbicides on the zooplankton community of a typical boreal forest lake was investigated using in situ enclosures and a replicated multiple-treatment-level experimental design. Concentration-dependent reductions of zooplankton abundance in response to hexazinone treatment were readily apparent, whereas metsulfuron methyl generated only marginal effects. Field-based, nonlinear regression estimates of EC50 (<6.0 mg L−1) for hexazinone-induced reduction of zooplankton abundance suggest that abundance of various zooplankton taxa may be expected following chronic exposure to environmentally realistic concentrations of this herbicide (i.e., approximately 1 mg L−1). Time to recovery was proportional to exposure level, with definite recovery observed at the 0.01- and 0.1-mg L−1 concentrations of hexazinone. The reduced magnitude of impact, as well as the relative timing of response and recovery in the zooplankton as compared to the phytoplankton community, was consistent with the postulate that hexazinone effects were secondary in nature, resulting indirectly from reductions in phytoplankton biomass and DO. The data provide evidence to suggest that chronic exposure to hexazinone concentrations approximating 1 mg L1− or above may result in reduced primary productivity in lentic ecosystems, which may in turn cause reductions in zooplankton abundance.