1. Metabolism of the 1",1"-dimethylheptyl analogue of cannabidiol (DMH-CBD) was studied using an isolated perfused rat liver preparation and in rat and dog urine. 2. Metabolites were identified using g.l.c.-mass spectrometry of the trimethylsilyl (TMS), methyl ester/TMS and [2H9]TMS derivatives. 3. In contrast with the metabolism of cannabidiol, the dimethylheptyl analogue gave low concentrations of metabolites in all media examined. 4. Four metabolites were found in the perfusion fluid. Two were identified as 6- and 7-hydroxy-DMH-CBD and the other two were found to be hydroxylated in the dimethylheptyl chain but at undetermined positions. 5. Five metabolites were identified in dog urine; these were the 6- and 7-mono-hydroxy and 6,7-dihydroxy derivatives of acids formed by one stage of beta-oxidation of the dimethylheptyl chain, and the 6- and 7-hydroxy derivatives of corresponding acids formed by loss of three carbon atoms from the chain. 6. Metabolic routes were very similar to those found earlier for cannabidiol.