An isolated rat hepatocyte preparation was used to study the cellular toxicity of cadmium and the protective effects of metallothionein on cadmium-induced toxicity. Exposure of primary suspension cultures of isolated rat hepatocytes to Cd2+ (0-35.7 microM) for 15 min resulted in a dose-dependent reduction in the synthesis of cellular proteins during a subsequent 6 h incubation. Such inhibition could not be correlated with cellular lethality or gross membrane damage. Pre-induction of metallothionein in hepatocytes by zinc treatment in vivo of donor rats protected hepatocytes in vitro from cadmium-induced inhibition of protein synthesis. The protective effects in zinc-pre-induced hepatocytes are not due to alterations in the level of total cellular cadmium, but could be accounted for by the redistribution of intracellular cadmium in the presence of high levels of zinc-metallothionein. The data suggest that metallothionein exerts its protective effect by a kinetic detoxification mechanism, i.e. a decrease in reactive intracellular cadmium.