3, 7-Dimethyl-1-(5-oxo-hexyl)-xanthine (BL 191) and its major urinary metabolites were examined for the inhibitory effect on the cyclic 3', 5'-nucleotide phosphodiesterase (PDE) activity and for the stimulating effect on lipolysis in rat epididymal fat pads in vitro. In addition, the relationship between these effects of BL 191 and plasma level of free fatty acid (FFA) was investigated in rats. The addition of BL 191 at 2 mM to the incubation mixture inhibited the activity of PDE about 50% and evoked a lipolytic activity 1.5-fold, and BL 191 and theophylline exerted approximately the same effects on PDE and lipolysis. Furthermore, the PDE inhibition and lipolytic response evoked by metabolites I and II possessing an oxohexyl group at the side chain were slightly less than those by BL 191, and metabolites III and IV having a carboxyl group at the side chain had no effects on PDE activity and lipolysis. When BL 191 was given orally, plasma FFA level was elevated in the blood taken 30 min after the administration, increased linearly until 1 hr after the administration when the level was nearly 1.6 times higher than the basal level, and remained at this level till 2 hr after the administration. The above-described results suggested that BL 191 along with its metabolites I and II by the oral administration of BL 191 inhibited the PDE activity and secondarily stimulated the lipolytic activity within the living body.