Cadmium toxicity and lead toxicity to four species of insects (Pteronarcys dorsata, Hydropsyche betteni, Brachycentrus sp. and Ephemerella sp.) one snail (Physa integra) and one amphipod (Gammarus pseudolimnaeus) were determined during 28-day exposures. The 28-day LC50 values for cadmium-exposed snails and lead-exposed amphipods were eleven and four times lower than the 7- and 4-day (96 h) values for these metals, respectively. Lowest effect concentrations obtained after 28 days for cadmium-exposed mayflies (Ephemerella sp.) and snails and lead-exposed amphipods were similar to those affecting fish exposed over their complete life cycle in water of similar quality. Lethal threshold concentrations were not observed for species exposed to either metal, indicating that possible effects could occur at lower concentrations during longer exposure periods. Cadmium and lead concentrations in the animals tested generally increased with increasing water concentrations and were up to 30,000 and 9,000 times greater than corresponding metal concentrations in the water.