We previously reported that circulating beta-endorphin levels are increased in obese hirsute women and that plasma immunoreactive insulin (IRI) levels are increased in proportion to the degree of hyperandrogenism in women with the polycystic ovary (PCO) syndrome. We, therefore, tested the hypothesis that endogenous opiates are at least partially responsible for the hyperinsulinemia and insulin resistance in this syndrome. In the first study, acute naloxone administration significantly reduced the plasma IRI response and IRI/glucose ratio in three euglycemic obese women with PCO and acanthosis nigricans (AN) and marked insulin resistance, but did not alter the glucose response. Naloxone had no effect on these parameters in the normal weight control subjects. In the second study, nalmefene, a new, orally active opiate antagonist, reduced IRI and the IRI/glucose ratio in four women with PCO-AN and marked hyperinsulinemia in a randomized, double blind, crossover protocol. We conclude that endogenous opiates are at least partially responsible for the hyperinsulinemia and insulin resistance in PCO-AN.