The mechanism of cadmium metallothionein (Cd-MT)-induced renal toxicity was studied in rats using urinary enzyme excretion as a marker for cellular damage. Animals were injected with either saline or Zn (20 mg of Zn as ZnSO4/kg b.wt.) at 0.5, 4 or 24 hr before injection of Cd-MT (0.3 mg of Cd as Cd-MT/kg b.wt. i.p.). Activities of two brush border enzymes, alkaline phosphatase (ALP) and gamma-glutamyl-transpeptidase (GGT), and a lysosomal enzyme, N-acetyl-beta-D-glucosaminidase (NAG), were measured in 24-hr urine collections. Urinary excretion of all three enzymes was increased significantly after Cd-MT injection. Both ALP and GGT excretions reached a maximum at 24 hr whereas NAG excretion reached peak values at 48 hr after Cd-MT injection. The excretion of all three enzymes decreased to the control level by the 3rd day. Zn pretreatment alone had no effect on urinary enzyme excretion. Pretreatment with Zn salts at 0.5 and 2 hr before Cd-MT injection did not show any difference in the urinary excretion of the enzymes as compared with the saline-treated controls. However, injection of Zn salts at 24 hr before Cd-MT injection resulted in a significant decrease in the excretion of the lysosomal enzyme NAG whereas both ALP and GGT excretions were unchanged. Extensive proximal tubular damage was observed morphologically in all the rats injected with Cd-MT, but the cellular damage was less in rats pretreated with Zn sulfate 24 hr before Cd-MT injection.(ABSTRACT TRUNCATED AT 250 WORDS)