I. IntroductionEssentially, two modalities of luteinizing hormone (LH) secretion have been identified in the rat. The basal mode is characterized by episodic bursts of pituitary LH discharge whose frequency and amplitude may vary considerably during the day in male (1, 2) and according to different phases of the estrous cycle in female rats (4). This low level of pulsatile LH secretion is established early in the juvenile period (5). With advancing age, subtle and progressive disintegration of several compo-nents of the LH episodes seems to account for the near cessation of rhythmicity in old rats (6). Additionally, in adult female rats, the periodic mode of LH secretion is interrupted on the afternoon of proestrus by fast, high amplitude pulses constituting the preovulatory LH surge lasting for 4–6 h (7–9). The cyclic pattern of LH secretion is fully established by the 5th week of pubertal life (5). Abnormalities in periodicity, onset, duration and magnitude of the preovulatory LH discharge during middle age precede reproductive senescence (10–14).