The present study examined the effects of 17 beta-estradiol benzoate (E2B) on the hypothalamic (HT) and anterior pituitary (AP) content of GnRH precursor (pro-GnRH-GAP), GnRH, GnRH-associated peptide (GAP), and substance-P (SP) during the various phases of the E2B-induced LH surge in cynomolgus monkeys. Changes in GnRH and GnRH-associated peptide (GAP) at both hypothalamic and AP levels were closely related at all times after E2B treatment. However, the pattern of change in the AP was very different from that in the HT. In the HT, pro-GnRH-GAP levels did not change significantly throughout the experimental period. In the AP, the pro-GnRH-GAP increased 48 h post-E2B treatment, the time of initiation of the LH surge. An 8-fold increase in AP GnRH occurred 30 h post-E2B treatment. There were no significant changes in the HT content of SP at any time after E2B treatment. However, there was a depletion of AP content by 48 h post-E2B, the time of the LH surge. These results demonstrate that E2 activates and deactivates in a coordinated manner the GnRH and SP systems of the HT-AP complex during initiation of the E2-induced LH surge. The observation that more significant changes occur in the AP than in the HT suggests that an important component of the E2 effect in inducing the LH surge may be directly at the AP level. This action involves changes in the contents of GnRH and SP in the AP.