BACKGROUND Evidence obtained from porcine cell cultures and experiments in laboratory animals indicates that transmembrane transporters may play a role in the distribution of the active morphine metabolite morphine-6-glucuronide (M6G). This was evaluated in a study in healthy volunteers. METHODS Ten subjects received an intravenous M6G infusion for 30 min at a dosage of 0.5 mg/kg body weight, leading to M6G plasma concentrations approximately two to three times higher than those observed with analgesic morphine doses in subjects with normal kidney function. In a randomized, double-blind, three-way crossover fashion, subjects received 800 mg quinidine for inhibition of P-glycoprotein; 500 mg probenecid for inhibition of other transporters, including organic anion transporter peptide, multidrug resistance-related protein, and organic anion transporter families; or placebo 1 h before the start of M6G administration. Plasma concentrations of M6G and pupil size were measured for 7 h. RESULTS Probenecid pretreatment resulted in a decrease in the clearance of M6G from 8.3 +/- 1 l/h to 6.7 +/- 1.3 l/h (factor of 0.8; P < 0.05 vs. placebo cotreatment). This was paralleled by an increase by a factor of 1.2 of the area under the miotic effect-versus-time curves (P < 0.05 vs. placebo). In contrast, quinidine pretreatment had no influence on the pharmacokinetics of M6G. CONCLUSIONS The active morphine metabolite is subject to transmembrane transport by transporters inhibited by probenecid in humans.