The metabolism of delta 1-tetrahydrocannabinol (delta 1-THC) by lung and liver of rat and guinea-pig have been studied by isolated perfused-organ techniques. Metabolites were identified by t.l.c. and mass fragmentography. Biotransformation in lungs showed a different quantitative pattern to the livers in both species. 4"-Hydroxy-delta 1-THC was a major metabolite in both rat and guinea-pig lung. 7-Hydroxy-delta 1-THC was another major metabolite in guinea-pig lung, but a minor one in rat lung. 6 alpha-Hydroxy-delta 1-THC and 6,7-dihydroxy-delta 1-THC were formed in small quantities in both species. Small amounts of 6 beta-hydroxy-delta 1-THC were detected in guinea-pig lung in addition to the metabolites mentioned. Livers of rat and guinea-pig formed 7-hydroxy-delta 1-THC as the most abundant metabolite together with minor amounts of 6 alpha-hydroxy- and 6,7-dihydroxy-delta 1-THC. 6 beta-Hydroxy-delta 1-THC was a minor liver metabolite in guinea-pig, and was found in rat only in trace amounts, together with delta 1-THC-6-one. No side-chain hydroxylated metabolites were detected in rat liver, while guinea-pig liver formed trace amounts of 3"-hydroxy- and 4"-hydroxy-delta 1-THC.