We investigated the functional expression of the plasma membrane serotonin transporter and the vesicular monoamine transporter in choriocarcinoma cells and normal trophoblasts. The RBL 2H3 cells, a rat basophilic leukaemia cell line, which express both transporters were used for comparison. The choriocarcinoma cells JAr and BeWo were found to possess the plasma membrane serotonin transporter as assessed by the presence of serotonin transport activity in intact cells that was Na(+)-dependent and was sensitive to inhibition by tricyclic and non-tricyclic antidepressants. The activity of the vesicular monoamine transporter in these cells was determined by measuring serotonin transport in digitonin-permeabilized cells. The transport in permeabilized cells was very slow, was not stimulated by ATP and was insensitive to inhibition by reserpine. Under similar conditions, the vesicular monoamine transporter activity was demonstrable in RBL cells, which was stimulated by ATP and was inhibitable by reserpine, bafilomycin A1 (an inhibitor of the V-type H(+)-pump) and carbonyl cyanide p-trifluoromethoxy phenylhydrazone (a protonophore which dissipates transmembrane H+ gradients). In corroboration with these findings, mRNA transcripts hybridizable to the vesicular monoamine transporter cDNA probe were detectable in RBL cells but not in JAr choriocarcinoma cells. Similarly, there was no evidence for the expression of the vesicular monoamine transporter as assessed by Northern blot analysis in normal trophoblasts which were maintained in culture to differentiate to form multinucleated syncytial cells. It is concluded that the trophoblasts and choriocarcinoma cells express the plasma membrane serotonin transporter but not the vesicular monoamine transporter.