Mercuric chloride was administered once ip to female Fischer 344 rats at doses of 0, 0.2, 0.6, and 1.8 mg/kg. Although there were no alterations in the urinary excretion of lactate dehydrogenase, significant elevations in the activities of urinary alkaline phosphatase, glutamicpyruvic transaminase and glutamicoxalacetic transaminase indicated that mercuric chloride was nephrotoxic. There was no evidence of hepatotoxicity as hepatic glucose-6-phosphatase and serum sorbitol dehydrogenase were essentially unaffected by mercuric chloride administration. Of the four phase II reactions measured, only the glucuronidation of chloramphenicol was diminished by treatment with mercuric chloride. Results from the in vivo studies on the metabolism of lindane, which indicated no change in the excretion of free or conjugated metabolites, were in close agreement with the in vitro data suggesting that the nephrotoxic effects of mercuric chloride do not alter the urinary excretion of the model substrate lindane.