To determine if the temporal pattern of LHsecretion changes during the days preceding the first preovulatory surge of gonadotropins, peripubertal female rats (28-37 days of age) were bled through indwelling jugular cannulae every 15 min for 3 h either in the mornings (900-1200 h) or the afternoons (1400-1700 h). In all pubertal stages plasma LH levels fluctuated in a pulsatile manner. In late juvenile rats (anestrous phase of puberty), the magnitude of the pulses in the afternoon was greater than in the morning and a drop in afternoon LH baseline levels became apparent. This pattern became established after day 28 and persisted throughout the late juvenile period and the early proestrous phase of puberty, a stage in which uterine fluid appears for the first time. During the first proestrus (late proestrous phase of puberty), morning LH levels fluctuated in a manner similar to that seen in anestrous and early proestrous animals. In the afternoon a preovulatory surge occurred, with LH levels increasing 40 times over morning values. On the next day (first estrus) the pattern of morning LH release was similar to that of prepubertal rats, but in the afternoon the greater magnitude of the pulses that had characterized the preovulatory period was not apparent. The results confirm earlier findings that tonic LH release does not increase gradually during prepubertal days as the hypothesis of the “gonadostat resetting” would have predicted. In addition, they demonstrate that during the late juvenile period (days 28-30), the afternoon mode of LH release changes, with the LH pulses acquiring greater amplitude in the presence of lower baseline values. The acquisition of this pattern of release may reflect the activation of facilitatory neural mechanisms controlling LH release and may be related, at least in part, to the changing peripubertal steroid milieu. It is conceivable that, in addition to the facilitatory effects exerted by PRL and GH on the prepubertal ovary, these previously unnoticed, subtle changes in the mode of LH release may play a role in determining the activation of the ovary that precedes the first preovulatory surge of gonadotropins. © 1981 by The Endocrine Society.