Cytopathological alterations in proximal (PS I, PS II) and distal segments (DS) of rainbow trout (Oncorhynchus mykiss) renal tubules following exposure to 0, 10, 20, 40, 80, and 160 micrograms/liter atrazine for 4 weeks were investigated by means of electron microscopy. Cellular responses were clearly dose-dependent with a gradual increase in variability and intensity of effects. Ultrastructural modifications in PS I and II were observed from 10 micrograms/liter atrazine, in DS from 20 micrograms/liter. In PS I, major changes included proliferation of smooth endoplasmic reticulum, atypical mitochondria and lysosomes, as well as gradual alterations of the apical plasmalemma. Typical changes in PS II cells were a proliferation of peroxisomes and ring- and cup-shaped mitochondria, as well as alterations in the basal labyrinth; DS cells were characterized by a proliferation of atypical mitochondria with longitudinally oriented cristae, disorganization of Golgi fields and vacuolization of the cell base. Results document that different segments of the renal tubule in rainbow trout react not only with different levels of sensitivity to atrazine exposure, but also in a segment-specific way. Moreover, comparison of effects induced by atrazine with those resulting from chronic exposure to the herbicide linuron revealed a distinct substance specificity in the reaction of different tubular segments.