LH-RH or its superactive analogues were infused into pituitary portal vessels of adult male rats for 1 min; pituitaries were then fixed for routine electron microscopy 1 min, 30 min and 3 h after infusion. A maximal increase of granule release at 2 min. and a maximal increase in synthetic activity (i.e., dilated cisternae of rough endoplasmic reticulum (RER) and Golgi apparatus (GA)) at 30 min, occurred in gonadotrophs in response to both LH-RH and the superactive analogues, but not to saline infusion. Only gonadotrophs from analogue infused pituitaries showed a high level of exocytosis and protein synthesis at 3 h. In addition, the superactive analogues typically stimulated formation of large vacuoles of RER at 30 min and 3 h that resembled those seen in "signet ring" cells following castration. Radioimmunoassay (RIA) values of serum LH supported these observations. The results of this study provide morphological evidence for a prolonged action of the superactive analogues of LH-RH on both gonadotropin secretion and synthesis under physiological conditions.