Evidence suggests that endogenous opioid peptides (EOP) inhibit pulsatile luteinizing hormone (LH) secretion during both the luteal and follicular phases of the ovine estrous cycle. Further data from sheep and other species indicate that the hypothalamus is the primary site of action for this EOP inhibition. The purpose of the following experiments was to determine which areas of the hypothalamus are involved in the EOP inhibition of pulsatile LH secretion. Regularly cycling ewes (n = 10) were stereotaxically implanted with guide tubes into the preoptic area (POA) and medial basal hypothalamus (MBH). Implants containing the EOP antagonist WIN 44,441-3 (WIN) were placed into each of these areas. Blood samples were collected at 12-min intervals for 3 h before and during WIN administration in the luteal phase and for 4 h before and during WIN administration in the follicular phase of the estrous cycle. During the luteal phase, WIN implants in either area increased (p less than 0.01) LH pulse frequency (POA 1.4 +/- 0.3/3 h before vs. 3.1 +/- 0.4/3 h during; MBH 1.1 +/- 0.2/3 h before vs. 2.8 +/- 0.5/3 h during). There was no effect on LH pulse amplitude. In contrast, during the follicular phase, WIN implants selectively increased (p less than 0.01) LH pulse frequency when implanted in the POA (3.2 +/- 0.4/4 h before vs. 5.2 +/- 0.6/4 h during) while increasing (p less than 0.05) only LH pulse amplitude when placed in the MBH (0.7 +/- 0.2 ng/ml before vs. 1.4 +/- 0.3 ng/ml during).(ABSTRACT TRUNCATED AT 250 WORDS)