Lactating female rats were infused with the serotonergic neurotoxin, p-chloramphetamine (PCA), via stainless steel cannulae implanted into either the 3rd or the lateral ventricle. Those mothers receiving the drug in the 3rd ventricle sustained a more severe depletion of hypothalmic serotonin (43% vs 23%) and a significantly greater incidence of pup mortality (98% vs 48%) than those receiving the drug in the lateral ventricle. Coincident with the depletion of hypothalmic serotonin was a reduction in serum prolactin (Prl). Subcutaneous (s.c.) injections of Prl into each group of females sharply reduced the number of pup deaths. It was concluded that hypothalmic serotonergic neurons mediate the suckling-induced release of Prl necessary for the maintenance of lactation.