The metabolism of (-)-delta1-tetrahydrocannabinol (delta1-THC) has been studied in the isolated perfused dog lung. After intravascular administration of [3H]-delta1-THC there was an overall biotransformation of 12%. Two major metabolites were isolated and identified as 3'-hydroxy-delta1-THC and 4'-hydroxy-delta1-THC. 7-Hydroxy-delta1-THC was also present together with small amounts of 6alpha-hydroxy-delta1-THC and 6beta-hydroxy-delta1-THC. An in vitro experiment using a dog liver microsomal preparation was also carried out and showed that the major metabolites were 6beta-hydroxy-delta1-THC and 6alpha-hydroxy-delta1-THC. 7-Hydroxy-delta1-THC and 1,2-epoxy-hexahydrocannabinol were also isolated together with small amounts of 3'-hydroxy-delta1-THC and 4'-hydroxy-delta1-THC. The side-chain hydroxylated compounds are hitherto undescribed metabolites of delta1-THC.