This study documents, for the first time, the temporal pattern of luteinizing hormone (LH) release and the relationship between plasma LH concentrations and testicular androgenic responses in young and aged male mice. Both LH and testosterone were measured in the same samples of blood plasma withdrawn at frequent intervals over 9 h from awake, mobile mice through intra-atrial cannulae. The results demonstrate unequivocally that LH and testosterone are discharged into the circulation in discontinuous pulses in this important animal model. Robust episodes of LH release occur at infrequent intervals and increments of circulating LH occasion time-delayed elevations of plasma testosterone. Thus, the frequency of LH release is the major factor determining peripheral concentrations of this gonadotropins. The obvious one-to-one coupling between intermittent LH discharges and testosterone secretion has significant functional and investigative implications. The reduction in circulating testosterone concentrations observed in old male mice is a consequence of fewer LH discharges, which is due most likely to the slowing of a neural GnRH pulse generator. However, age-related declines in copulatory behavior were not associated with altered patterns of hormone secretion. Hence, we suggest that deficits in sexual behavior and episodic LH release in old males result from neural senescence rather than diminished testicular support of reproduction.