Blood from the external carotid artery and from the external jugular vein were collected simultaneously, and luteinizing hormone (LH) concentrations were measured in order to determine the temporal pattern of the large episodic pulses of LH secretion which are characteristic of ovariectomized sheep. Collection of paired arterial and venous samples at 5-min intervals did not provide adequate resolution of arteriovenous LH changes which accompany these pulses. Therefore, a continuous blood sampling system was developed which permitted collection of paired arterial and venous samples over 20-second intervals throughout a sampling period of approximately 45 min. One large pulse of LH secretion was detected in 6 of 9 trials, and each pulse was characterized by a brief period (2–6 min) when venous concentrations of LH were much higher than corresponding arterial concentrations. Periods of considerable release of LH from the head were usually followed by a period when arterial LH levels were greater than the venous levels, indicating that LH was being removed from the circulation by the tissues of the head. Administration of synthetic GnRH pulses (17.5–35 ng) through the arterial cannulae of halothane-anesthetized ewes produced in different trials arteriovenous LH concentration differences ranging from no discharge of LH to discharges much larger and more prolonged than those occurring spontaneously in conscious ewes. These results suggest that individual episodes of spontaneous pulsatile discharge of LH in ovariectomized sheep last 2–6 min and probably result from episodic secretion of LH-releasing hormone by the hypothalamus. Profiles of circulating LH also appear to be modified by extravascular sequestration of LH.