To assess a possible regulatory influence of opioids upon anterior pituitary function in the chimpanzee, we evaluated the effects of the specific opiate receptor antagonist naloxone and the agonistic enkephalin analog [D-Ala², MePhe⁴,Met(o)-ol]enkephalin (FK 33–824; Sandoz) on serum levels of PRL, cortisol, FSH, and LH. Under ketamine anesthesia, the following were administered by iv injection during the early follicular phase of successive menstrual cycles in nine female chimpanzees: naloxone (10 mg; n = 7) or saline vehicle (n = 7) randomly assigned in the first two cycles, FK 33–824 0.25 mg (n = 3) in the third cycle, FK 33–824 0.50 mg (n = 4) in the fourth cycle, and FK 33–824 (0.50 mg) immediately preceded by naloxone (10 mg; n = 4) in the last cycle. Five pretreatment and 12 posttreatment serum samples were obtained at 10- to 15- min intervals for subsequent RIA. Naloxone caused a significant reduction in PRL levels from a pretreatment mean of 29.3 ng/ ml to a mean of 11.1 ng/ml at 180 min. Values from 60–180 min were significantly below the saline control group at comparable times. A dose-related increment in PRL levels was seen after FK 33–824 administration, with mean peak values at 30 min of 61.0 and 92.3 ngsol;ml after the low and high doses, respectively. Naloxone pretreatment markedly attenuated the response to high dose FK 33–824. Cortisol levels rose in all groups throughout the study period, a presumed effect of the ketamine anesthesia. Compared to the saline group, no effects of FK 33–824 were observed. Naloxone, given alone or with FK 33–824, had a small, but significant, stimulatory effect on cortisol from 60–120 min posttreatment compared to the control group. Naloxone caused a significant increment in LH levels from a pretreatment mean of 11.7 μg/dl to a peak of 19.1 μg/dl at 30 min and in FSH level from 33.2 μ/dl before therapy to 40.0 μg/dl at 45 min. There was no influence of FK 33–824 on gonadotropin levels, although the high dose did blunt the response to naloxone. Taken together, these effects suggest that opiate agonists and endogenous opioid pathways may modulate anterior pituitary function in the chimpanzee, as in man.