The in vivo metabolism of delta1-tetrahydrocannabinol (delta1-THC) was further investigated in the rabbit after i.v. administration. Nine acidic metabolites were isolated from a previously not investigated fraction of the urine and identified by gas chromatography-mass spectrometry and by proton magnetic resonance spectroscopy. The major metabolites were side-chain hydroxylated monocarboxylic acids. Three side-chains monocarboxylic acids hydroxylated in allylic positions in the isoprene moiety were also characterized. The metabolites 4''-hydroxy-delta1-THC-7-oic acid and 7-hydroxy-4'',5''-bisnor-delta1-THC-3''-oic acid were hitherto not identified. An earlier described dicarboxylic metabolite was present in high concentration. Further, the identity of an O-glucuronide as an in vivo urinary metabolite of delta1-THC was here for the first time unambiguously established by m.s. and p.m.r.