delta 9-tetrahydrocannabinol (THC), 11-nor-9-carboxy-delta 9- tetrahydrocannabinol (THC-COOH), and its O-ester glucuronide were measured in plasma by GC/MS and in urine by GC/MS and enzyme immunoassay after frequent and infrequent marijuana users were given 5 mg THC intravenously. Plasma THC concentrations were detectable 4-5 h after infusion using solid-phase columns for drug extraction and gas chromatography/mass spectrometry (GC/MS) for detection and quantification. THC-COOH and its O-ester glucuronide were analyzed for 12 days. Concentrations were higher in the plasma and urine of frequent marijuana users. The ratio of THC-COOH to its O-ester glucuronide in plasma was greater than 2 in both frequent and infrequent users 2 to 30 min postinfusion. Ratios for the subsequent 12 days were less than 2. A ratio of less than 1 for total THC-COOH/THC occurred only before 45 min for both frequent and infrequent users. Physiological, psychological, and pharmacokinetic data revealed few differences between frequent and infrequent marijuana users.