The pattern of GnRH secretion during the follicular phase of the estrous cycle of sheep is characterized by an initial marked change in episodic secretion (increased frequency and decreased amplitude) followed by a massive and sustained discharge-the preovulatory GnRH surge. Studies employing a physiological model for the follicular phase have revealed that estradiol has profound and complex feedback effects on GnRH release during the preovulatory period. These include both quantitative effects on pulses (stimulation of frequency, inhibition of amplitude) and qualitative effects (altering pulse shape, stimulating interpulse secretion), in addition to inducing a preovulatory GnRH surge. In stimulating the surge, estradiol causes a highly characteristic change in the minute-to-minute pattern of GnRH in hypophyseal portal blood. Initially, a strictly episodic pattern gives way to one in which GnRH is consistently elevated between pulses. Then, following enhancement of both pulsatile and interpulse components, GnRh becomes extremely high and variable for the majority of the surge. From this point, a regular and well organized pulse pattern is not apparent. The characteristic time course of GnRH at surge onset provides insight into possible mechanistic changes in the GnRH neurosecretory system. Such changes include quantitative and qualitative alterations in the pulse generating mechanism, recruitment of a surge specific population of GnRH neurones, morphologic alterations in GnRH neurones and neighboring cells, and changes in efficiency or route of delivery of GnRH from its site of release to the portal vasculature. These possibilities, while untested and speculative, provide a conceptual framework for future research.