To reach its target cells, the antiepileptic drug valproate has to cross both the intestinal epithelial barrier and the blood-brain barrier in intact form as well as in sufficient amounts. This study was performed to characterize the epithelial transport of valproate at intestinal (Caco-2) and at blood-brain barrier (RBE4) cells. At both cell types, uptake of [(3)H]valproate was independent of inwardly directed Na(+), Ca(2+), Mg(2+), K(+) or Cl(-) gradients. Uptake was, however, strongly stimulated by an inwardly directed H(+) gradient. The cells accumulated valproate against a concentration gradient and the uptake rate of valproate was saturable with K(t) values of 0.6 and 0.8mM. At Caco-2 cell monolayers, the total apical-to-basolateral flux of [(3)H]valproate exceeded the basolateral-to-apical flux 14-fold. Various monocarboxylic acids like salicylate, benzoate, acetate, propionate, butyrate, hexanoate, diclofenac and ibuprofen inhibited [(3)H]valproate uptake at both cell types. Lactate and pyruvate inhibited valproate uptake at RBE4 cells but not at Caco-2 cells. We conclude that valproate is accumulated in intestinal cells against a concentration gradient by the activity of a specific H(+)-dependent DIDS-insensitive transport system for monocarboxylates not identical with monocarboxylate transporter 1 (MCT1). The passage of valproate across the blood-brain barrier is very likely mediated by MCT1.