The hypothesis that serotonin (5-HT) influences the onset of differentiation (cessation of division) of prospective 5-HT target neurons during embryogenesis was tested by administering the 5-HT depleting drug p-chlorophenylalanine (pCPA) to pregnant rats and dating the time of last cell division for fetal neurons using long survival 3H-thymidine autoradiography. PCPA specifically retarded the onset of neuronal differentiation in brain regions known to contain 5-HT terminals or to have a high 5-HT content in the adult (5-HT target cells). Conversely, the 'stress' of daily vehicle injections accelerated onset of differentiation in these regions, possibly due to stimulation of 5-HT synthesis by glucocorticoids. These data support the model of 5-HT as a humoral signal for the differentiation of cells later recognized by 5-HT neurons as appropriate targets for synaptogenesis.