Previous investigations have suggested that probenecid (PRB) alters the in vivo disposition of valproic acid (VPA), perhaps by inhibiting hepatic formation of valproate glucuronide (VG). Because VPA and PRB bind moderately to plasma proteins, protein binding also is a potential locus of interaction. The purpose of this investigation was to determine whether in vitro systems could accurately predict PRB-associated perturbations in the hepatobiliary disposition of VPA and VG in vivo. VPA and PRB were coadministered to rats for 60 min at various infusion rates to examine steady-state VPA disposition. PRB did not alter the binding of VPA in serum or hepatic cytosol. However, PRB decreased the apparent intrinsic clearance of VPA (1.81 +/- 0.58 versus 1.23 +/- 0.23 ml/min; P =.025) by competitively inhibiting VPA elimination. In a separate study, rat hepatic S9 fractions were incubated with VPA (7.2-721 microg/ml) and PRB (0-2850 microg/ml). VG formation (V(max) = 0.80 +/- 0.06 microg/min/mg of protein; K(m) = 173 +/- 28.8 microg/ml) was impaired by PRB in a competitive manner (K(i) = 876 +/- 559 microg/ml), consistent with the in vivo data. Despite inhibition of phase II metabolism of VPA to VG by PRB, the VG biliary excretion rate at similar unbound VPA concentrations in hepatic cytosol was not lower in PRB-treated rats. These results indicate that VG disposition in the presence of PRB cannot be predicted accurately based solely on in vitro inhibition of glucuronidation and emphasize the complexity of processes associated with the hepatobiliary system.