The morphological changes in renal proximal tubules of Sprague Dawley rats given acute or chronic exposure to cadmium were analysed. Cadmium chloride was administered either by five subcutaneous injections of cadmium at 2 mg kg-1 body weight or in drinking water at 100 micrograms ml-1 for 39 weeks. The mean cadmium concentration in the kidneys of these rats was 45 and 102 micrograms g-1 wet weight respectively. The rats were anaesthetized and the kidneys were fixed by perfusion and processed for electron microscopy. Proximal tubule profiles were larger in the acute exposure rats. The brush border in both groups of treated rats was shorter and there were focal areas of loss of microvilli. The surface density of microvillus membrane per unit cell volume was reduced by 25% and 19% for chronic and acute dosed rats respectively. There were few significant changes in organelles detected by morphometric analysis of the entire kidney tissue, however there was a reduction in volume density of lysosomes following chronic exposure and individual necrotic cells and distorted nuclei were observed. The morphological changes observed in chronic and acute dosed rats were consistent with a primary site of toxic insult on the apical plasma membrane. There appeared to be no evidence for change in fluid and electrolyte homeostasis in the epithelium. The results suggest that the cadmium may produce a selective deficit of transport mechanisms for macromolecules.