Serotonin plays a trophic role in brain cell differentiation. In this study, expression of the serotonin presynaptic transporter protein, which regulates the extracellular serotonin concentration, was measured with [3H]paroxetine in rats exposed to dexamethasone or cocaine prenatally. Within 24 h of a single dose of dexamethasone, significant increases were seen in fetal brain, and the effect persisted into the postnatal period. Chronic prenatal cocaine exposure elicited similar changes. These data indicate that exposures to apparently disparate drugs can elicit similar endpoints that may lead to behavioral teratogenesis.