In adult rats 6 mg kg-1 body wt. cisplatin given i.p. was nephrotoxic. Four days of i.p. treatment with 40 mg kg-1 body wt. methimazole, which started 1 day before CP, prevented increases in blood urea nitrogen and in the renal excretion of proteins. Furthermore, methimazole treatment reduced the oliguric effect of cisplatin and the depression of renal sodium excretion. However, it had no effect on the increased formation of lipid peroxides in cisplatin-damaged kidneys, although repeated treatment with methimazole enhanced the renal glutathione content. Methimazole acts as a radical scavenger, maintaining the glutathione pool in the kidney.