The present study provides the first autoradiographic evidence of age-dependent regional changes in the density of serotonin (5-HT) transporters in offspring following prenatal exposure to fluoxetine. Pregnant rats received either saline or fluoxetine (10 mg/kg, s.c.) daily from gestational day 13 through 20. The density of [3H]citalopram-labeled 5-HT transporters was determined in forebrain regions and in midbrain raphe nuclei of prepubescent and adult male offspring. Brain regions representing integral components of the limbic system were particularly sensitive to the prenatal treatment. For example, prenatal fluoxetine exposure significantly altered the density of 5-HT transporters in subregions of the hypothalamus (dorsomedial nucleus, -21%; lateral hypothalamus, +21%), hippocampus (CA2, +47%; CA3, +38%), and amygdala (basolateral nucleus, +32%; medial nucleus, +44%) in prepubescent offspring. However, 5-HT transporter density in the dorsal and median raphe was unaltered in this same group of offspring. In adult offspring, 5-HT transporter densities, in all brain regions examined, were not significantly altered by prenatal exposure to fluoxetine. The present study also identifies significant age-related differences in 5-HT transporter densities between prepubescent and adult control offspring. For example, in adult control offspring, densities of 5-HT transporters were significantly greater in the cingulate cortex (+33%), basolateral amygdala (+58%), and CA1 area of the hippocampus (+78%); but significantly lower in the temporal cortex (-65%) and median raphe (-25%). The age-dependent and site-specific alterations in the density of 5-HT transporters suggests that either 5-HT innervation and/or 5-HT neuron function in various forebrain regions may be altered by prenatal exposure to fluoxetine.