The objective of this study was to investigate the impact of probenecid on the blood-brain barrier (BBB) transport of morphine-3-glucuronide (M3G). Two groups of rats received an exponential infusion of M3G over 4 h to reach a target plasma concentration of 65 microM on two consecutive days. Probenecid was co-administered in the treatment group on day 2. Microdialysis was used to estimate unbound M3G concentrations in brain extracellular fluid (ECF) and blood. In vivo recovery of M3G was calculated with retrodialysis by drug, preceding the drug administration. The BBB transport was modelled using NONMEM. In the probenecid group, the ratio of the steady-state concentration of unbound M3G in brain ECF to that in blood was 0.08+/-0.02 in the absence and 0.16+/-0.05 in the presence of probenecid (P=0.001). In the control group, no significant difference was found in this ratio between the 2 days (0.11+/-0.05 and 0.10+/-0.02, respectively). The process that appears to be mainly influenced by probenecid is influx clearance into the brain (0.11 microl min(-1) g-brain(-1) vs 0.17 microl min(-1) g-brain(-1), in the absence vs presence of probenecid, P:<0.001). The efflux clearance was 1.15 microl min(-1) g-brain(-1). The half-life of M3G was 81+/-25 min in brain ECF vs 22+/-2 min in blood (P<0.0001). Blood pharmacokinetics was not influenced by probenecid. In conclusion, a probenecid-sensitive transport system is involved in the transport of M3G across the BBB.