The effect of microinjection of catecholamines into the 3rd brain ventricle on plasma gonadotropins and prolactin (Prl) titers was evaluated in estrogen and progesterone-primed, ovariectomized (OEP) rats which are a particularly sensitive test animal for LH-RH and the responses compared to those obtained in ovariectomized (OVX), unprimed animals. Intraventricular injection of isotonic saline had no effect on plasma gonadotropins or Prl. The injection of dopamine (DA) into the 3rd ventricle elevated plasma LH in the OEP rats at doses of 4 or 20 µg, but the elevation obtained with the 10 µg dose was not significant. A greater effect was obtained with 20 µg doses of both norepinephrine (NE) and epinephrine (E), with E being the most effective of the 3 catecholamines. Apomorphine (APO) also elevated plasma LH following its injection into the 3rd ventricle in the OEP rats. Since these animals were etherized for removal of blood samples and microinjections into the ventricle, a group of animals was used with in-dwelling cannulae in the external jugular vein. In these animals, DA also elevated plasma LH. In OVX animals, the intraventricular injection of DA was without effect on plasma LH, but small transient increases in plasma LH were obtained with 20 µg doses of NE or E. The action of DA in the OEP rat was not prevented by the prior injection of diethyldithiocar-bamate (DDC) which inhibits NE synthesis. Dopamine, apomorphine, and another DA receptor-stimulating drug, Piribedil (ET-495), lowered plasma Prl following their intraventricular injection in all conditions. There were no effects of any of the drug treatments on plasma FSH. It is concluded that intraventricular DA can stimulate LH release in the steroid-primed rat but that NE and E appear to be more effective in this regard. Epinephrine was the most effective catecholamine in stimulating LH release. Dopamine clearly