The in vivo metabolism of delta1-tetrahydrocannabinol (delta1-THC) was investigated in the rabbit after i.v. administration. Thirteen acidic metabolites were isolated from rabbit urine and identified by gas chromatography-mass spectrometry and by proton magnetic resonance spectroscopy. One additional metabolite was tentatively identified. All but three were new metabolites and all but one were oxidized in the pentyl side chain. The metabolites included dicarboxylic acids, monocarboxylic acids and mono- or dihydroxylated derivatives thereof. However, the dicarboxylic acid metabolites were the most prominent.