Uptake of the H2-receptor antagonist, cimetidine, into syncytial microvillus membrane vesicles of human term placenta was investigated to clarify whether an active transport mechanism can be responsible for the observed barrier of the human placenta for cimetidine. Imposition of an outwardly directed H(+)-gradient stimulated cimetidine uptake, resulting in a small transient overshoot. The H(+)-gradient-dependent peak uptake was decreased under voltage-clamped conditions by carbonyl cyanide p-trifluoromethoxy-phenylhydrazone, suggesting the presence of an organic cation-proton exchange mechanism. Uptake was partially, but significantly, inhibited by organic cation transport inhibitors, H2-receptor antagonists and several other cationic drugs, providing further evidence for mediated uptake. H(+)-gradient-dependent cimetidine uptake was saturable and characterized by a low-affinity (Km) of 6.3 mM and Vmax of 17.5 nmol/mg protein/10 sec. We conclude that the system cannot play an important role in the barrier function of the human placenta in the transport of cimetidine. Rather than active transport, other factors, as for instance the degree of ionization of cimetidine at physiological pH, seem to be a more likely explanation for the low clearance of cimetidine across the human placenta.