Freshly isolated pituitaries from female rats between the day of birth and 14 days were challenged with GnRH in a perifusion system. At the day of birth GnRH significantly stimulated GH release but had no effect on PRL release, whereas PRL secretion was strongly stimulated by TRH. GnRH-stimulated PRL release was seen from day 3 up to about day 9 after birth, with a maximum in the magnitude of the response on day 5. GH release showed also a maximal response to GnRH on day 5. However, although stimulated PRL release declined rapidly at the end of the GnRH pulse, GH release did not and even increased over a period of about 20 min. The latter secretion dynamics are consistent with previous observations in reaggregate cell cultures that the GnRH effect on GH release is dual (stimulation and inhibition). The poststimulus rise of GH release was most pronounced with pituitaries of 7- and 9-day-old rats. Finally, in pituitaries of 14-day-old rats the effect on GH release was largely diminished. The present data show that GnRH affects the secretion of lactotrophs and somatotrophs in intact rat pituitary glands only during a restricted period of early postnatal life in a cell-type specific manner and shed new light in establishing the role of GnRH in modulating PRL and GH release.