A metabolic relationship between the liver and kidney exists in combination with the urea cycle. In patients with acute renal or hepatic insufficiency, certain variations among several guanidino compounds are relevant to the condition of the liver or kidney. An experimental model of rats exposed to inhaled halothane was evaluated for metabolic changes in the liver and kidney by measuring guanidino compound levels using high performance liquid chromatography. As a result, no significant difference in serum ornithine carbamoyltransferase (OCT) activities were observed among all groups. In the liver, concentrations of creatine (CR) and guanidinobutyric acid (GBA) were significantly elevated immediately after inhalation of halothane without any accompanying changes in guanidinoacetic acid (GAA) and argnine (ARG). In the kidney, ARG and GAA were significantly decreased without any accompanying changes in CR and GBA. No significant difference in serum GC levels was observed among the groups. The inhalation of halothane appers to change the concentrations of GAA, ARG and CR in the liver and kidney. It also appers that inhalation of halothane caused GBA to appear in the blood, indicating a possible association of halothane with hepatotoxicity.