Using a perfused innervated rat heart model, we studied the sex difference in the sympathetic nerve stimulation-induced norepinephrine release and its presynaptic alpha 2-adrenergic inhibition in normoxic and ischemic conditions. During normoxic perfusion, the alpha 2-adrenoceptor antagonist rauwolscine resulted in a higher overflow of norepinephrine during nerve stimulation in females than in males (p less than 0.05). This more marked potentiation of norepinephrine overflow in females was accompanied by an increased chronotropic and inotropic response (p less than 0.01). During early stop-flow ischemia neural norepinephrine overflow was lower in female than in male hearts (p less than 0.005). Rauwolscine enhanced norepinephrine overflow more in females than in males (p less than 0.05), thereby eliminating the initial difference in norepinephrine overflow during ischemia between the two sexes. Ovariectomy attenuated the presynaptic alpha 2-adrenergic inhibition of norepinephrine release compared with sham-operated females (p less than 0.02). No sex difference was found in either cardiac norepinephrine content or nonexocytotic norepinephrine overflow induced by a 40-minute period of stop-flow ischemia. Thus, presynaptic alpha 2-adrenergic inhibition of myocardial norepinephrine release is greater in female than in male rats. This difference persists into the early phase of ischemia and is largely responsible for the lower neural norepinephrine release in the female heart. Female hormones may increase presynaptic alpha 2-adrenergic activity in the heart.