The role of endogenous thyrotropin-releasing hormone (TRH) in the control of pituitary thyroid-stimulating hormone (TSH) and growth hormone (GH) secretion was studied during postnatal maturation in female Wistar rats. Half of the sucklings in each litter was treated intraperitoneally with either specific rabbit antiserum against TRH or normal rabbit serum (0.1-0.3 ml according to age). All animals were decapitated after 2 h. The presence of anti-TRH activity was checked as a binding of labelled TRH with plasma of the experimental animals. Immunoneutralization of endogenous TRH resulted in a decrease of plasma TSH in 3- to 15-day-old female pups as compared to control littermates. No effect of TRH antibody injection was seen at the ages of 1, 21, 30 and 70 days despite the presence of excess antibody in the plasma. A profound effect of TRH antibody on plasma TSH was seen again at the age of 100 days. Plasma GH in the same animals exhibited a paradoxical increase after TRH immunoneutralization at the age of 5 and 8 days, a decrease was found at the age of 21 days. It was concluded that hypothalamic TRH control of TSH secretion matures early in Wistar rats. Hypothalamic secretion of TRH at the ages of 1, 21, 30, and 70 days is low and(or) its role in TSH regulation is masked by other regulating factors. TRH may play a dual role in the regulation of GH secretion during the postnatal period.