We investigated the characteristics of cocainelike binding sites in rat placenta using [¹²⁵I]RTI-55. [³H]paroxetine binding and immunocytochemical staining for serotonin [5-hydroxytryptamine (5-HT)] and for the 5-HT transporter were also used to obtain evidence for rat placental 5-HT uptake. [¹²⁵I]RTI-55 saturation analyses with membranes from normal gestational day 20 placentas yielded curvilinear Scatchard plots that were resolved into high- and low-affinity components (mean dissociation constants of 0.29 and 7.9 nM, respectively). Drug competition studies with various monoamine uptake inhibitors gave rise to complex multiphasic displacement curves, although the results obtained with the selective 5-HT uptake inhibitor citalopram suggest that the 5-HT transporter is an important component of placental high-affinity [¹²⁵I]RTI-55 binding. The presence of a rat placental 5-HT uptake system was additionally supported by the [³H]paroxetine binding experiments and by the presence throughout the placenta of immunoreactivity for 5-HT and the 5-HT transporter. Immunostaining with both antibodies was most intense in the junctional zone, whereas the density of [¹²⁵I]RTI-55 binding sites was greater in the placental labyrinth. This discrepancy may be due to the fact that [¹²⁵I]RTI-55 appears to be labeling additional cellular components besides the 5-HT transporter. The presence of cocaine- and antidepressant-sensitive 5-HT transporters in the placenta has important implications for the possible effects of these compounds on pregnancy and fetal development.