The timing of the episodic secretion of luteinizing hormone (LH) in ovariectomized ewes was investigated with respect to an ultradian rest-activity cycle. Individual sheep were enclosed in a chamber where their physical activity was continuously monitored, and blood samples for LH analysis were collected at 5-min intervals through a cannula extending to the exterior of the chamber. Concurrent profiles of motor activity and plasma LH during 5-hour sampling trials were each analyzed for rhythmicity by power spectral analysis. Temporal relationships between rhythms in motor activity and plasma LH were examined by cross-spectral analysis. Both activity and plasma LH levels tended to fluctuate rhythmically, and the occurrences of rhythmicity for LH and for activity were correlated. The periods (reciprocal of frequencies) for the LH rhythms and activity rhythms were highly correlated within individual trials, and across trials they both averaged 36 min. Cross-spectral analysis indicated that plasma LH tended to be elevated during times of either greatest or least motor activity. These results suggest a relationship between the rhythmic secretion of LH and the rest-activity cycle which may be based on a mechanism of independent entrainment to a common timing rhythm.