Long-retained metabolites of .DELTA.9-tetrahydrocannabinol (.DELTA.9-THC) and .DELTA.8-tetrahydrocannabinol (.DELTA.8-THC) previously detected in the liver, spleen, fat and bone marrow of the rat after i.v. or i.p. injections of 14C-.DELTA.8-THC or 14C-.DELTA.9-THC, were identified as fatty acid conjugates of 11-hydroxy-.DELTA.9-THC and 11-hydroxy-.DELTA.8-THC. Characterization of the metabolites was facilitated by their large-scale in vitro production from 11-hydroxy-.DELTA.9-THC and 11-hydroxy-.DELTA.8-THC with a rat liver microsomal enzyme system. Conjugation of long-chain fatty acids to drugs or their hydroxylated metabolites may have considerable biological significance.