A mixture (DA-C) of dopamine hydrochloride and cholesterol in a ratio of 2 to 1 or 5 to 1 by weight was implanted into different hypothalamic regions of the rat and allowed to persist for more than two weeks. The estrous cycles and ovarian histology were examined to determine effects of the treatment. When the tip of the cannula containing DA-C (2 to 1) hit the posterior median eminence (ME) and a small part of the arcuate nucleus (AN), the rats showed prolonged estrus. The ovaries of these rats showed big follicles and no corpora lutea. When the tip of the cannula of DA-C (5 to 1) hit the posterior ME and a small part of the AN, the rats showed prolonged diestrus. The follicles and corpora lutea were small in the ovaries of these rats. Under our experimental conditions, DA seems to inhibit LH and FSH release at the level of the ME. When the tips of the cannulae containing DA-C were placed in AN, n. ventromedialis, n. premamillaris ventralis, n. premamillaris dorsalis, n. dorsomedialis pars ventralis or adenohypophysis, the rats showed cycles with or without minor irregularities.