Catecholamines were infused through an atrial cannula in unanesthetized rats on the afternoon of proestrus and blood was withdrawn through a second cannula for radioimmunoassay of LH and prolactin. Infusion of epinephrine, but not of norepinephrine or dopamine, blocked spontaneous pituitary LH release and ovulation. Ultimately, this effect appears to be exerted on the brain and not on the pituitary or through changes in pituitary blood flow. Pituitary LH release in response to exogenous LHRH, when administered in an amount that simulated the proestrous LH surge in phenobarital-treated rats, was unaltered by epinephrine infusion. In addition, epinephrine infusion did not alter the timing of the rise in plasma prolactin. Infusion of dopamine blocked the spontaneous rise in plasma prolactin and depressed basal prolactin levels. After the end of infusion, plasma prolactin rose rapidly. Infusion of norepinephrine or epinephrine partially suppressed the prolactin rise but only after 2 h of infusion. RESULTS 1) point out the possibility that chronic release of epinephrine from the adrenal medulla may be involved in phenomena in which "stress" inhibits reproductive function; and 2) are consistent with the view that dopamine, but not norepinephrine, may be PIF.