Adult male Wistar rats were given 50 ppm Cd in drinking water over a period of 1-24 weeks. The rats were killed and the cadmium concentration of whole blood, blood plasma red cells, liver and kidneys estimated. The plasma metallothionein concentration was measured by radioimmunoassay. Kidney samples were taken for light, transmission and scanning electron microscopic examination. The accumulation of cadmium in the tissues was shown by a linear increase with time, after exposure for 12 weeks. Plasma Cd concentrations showed a clear increase after 3 weeks and preliminary investigation suggests that most is present as Cd-thionein. Early pathological changes in the rat kidney were seen around the 4-6 week period which coincided with the distinct rise in plasma Cd. At 12 weeks, signs of tubular necrosis, interstitial fibrosis and glomerular epithelial cell hypertrophy were present in small areas of the cortex. By 24 weeks, the renal cortex showed clear evidence of tubulo-interstitial nephritis at a Cd concentration of 60 micrograms Cd/g wet weight.