Levels of glutathione, glutathione reductase and glutathione S-transferase activities in rat lung and liver have been investigated. After perfusing the lung to remove contaminating blood, this organ was found to have an apparent concentration of glutathione (2mM) which is approx. 20% of that found in the liver. Both organs contain very low levels of glutathione disulfide. Neither phenobarbital nor methylcholanthrene had a significant effect on the levels of reduced glutathione in lung and liver. In addition, the activities of some glutathione-metabolizing enzymes--glutathione reductase and glutathione S-transferase activity assayed with four different substrates--were observed to be 5-to 60-fold lower in lung tissue than in the liver.