The effects of low-level lead (20 micrograms/liter) and/or cadmium (2 micrograms/liter) exposure on the structure and function of different organ systems of the freshwater crayfish. Astacus astacus L. (Crustacea: Decapoda) were estimated by several biochemical and histochemical methods. The animals were incubated during 10 weeks (max.) at a temperature of 10 degrees C and a normal diurnal rhythm. Lead accumulated in high amounts especially in the digestive gland, carapax, and gills, whereas the hindgut and musculature exhibited very low lead levels. Cadmium accumulated particularly in the digestive gland and gills. Lead and cadmium levels were definitely lower in the digestive gland, gills, and carapax of animals incubated in water containing a double, i.e., lead and cadmium load, than in animals kept in water containing only one of these heavy metals. Histochemically both metals could be visualized in a typical distribution within the tissues, such as the carapax, digestive gland, or gills. After several weeks of poisoning, all organs, but especially the digestive gland, showed severe structural impairment. The activities of oxidative enzymes in the digestive gland and gills were significantly lowered after 2 weeks of incubation. Enzyme histochemical evaluation demonstrated changes of reaction intensities within the organs as compared to the controls. GSH S-transferase activities and GSH contents were also distinctly decreased following lead and/or cadmium intoxification. The histochemical demonstration of SH and S-S groups exhibited a stronger staining reaction after 10 weeks of exposure, especially in digestive gland and gills. The results obtained are discussed in view of the specific impairment of function of the organ systems studied, as related to the typical biology of the animal species tested.