The neuroendocrine role of endogenous ligands for the excitatory amino acid receptor subtype known as the NMDA receptor was investigated by administering the NMDA receptor antagonist MK-801 to ovariectomized (ovx) and estradiol-treated sheep. Repetitive administration of MK-801 at intravenous (iv) dosages of 0.1 mg/kg to untreated ovx ewes did not affect the episodic profiles of luteinizing hormone (LH) release, but each injection of MK-801 abruptly stimulated release of prolactin (PRL) demonstrating the effectiveness of the dosage. Injection of estradiol-17beta (50 microg/ewe) to ovx ewes produced the expected biphasic LH response; an initial suppression followed by a surge-like LH increase together with an elevated basal secretion of PRL. Injections of MK-801 occurring 9 and 11 h after estradiol-17beta injection rapidly and transiently increased serum LH in a very unexpected response. However, these same MK-801 injections also resulted in decreased serum LH 14-17 h after estradiol-17beta by delaying the onset of the surge-like release of LH. Estradiol-17beta administration itself elevated basal release of PRL to serum concentrations observed after repetitive MK-801, and further injection of MK-801 no longer transiently increased serum PRL as it had done prior to estradiol-17beta treatment. In summary, antagonizing the endogenous excitatory amino acid ligands of the NMDA receptor with MK-801 did not alter either the timing or magnitude of the putative episodic discharges of gonadotropin-releasing hormone (GnRH) which in turn cause the episodic releases of pituitary LH in ovx sheep. Estrogen administration created a transient neuroendocrine environment in which antagonism of these endogenous ligands was stimulatory to abrupt discharge of GnRH and thereby to acute release of LH. Antagonism of NMDA receptors in untreated ovx ewes stimulated release of PRL suggesting that the endogenous NMDA ligands were probably stimulatory to the release of a PRL-inhibiting neurohormone such as dopamine.