A total of 17 women affected by polycystic ovarian disease (PCOD) were studied to evaluate the involvement of endogenous opioids in the pathophysiology of the hyperinsulinism in PCOD by administering naltrexone, an oral opioid antagonist. An oral glucose tolerance test (OGTT) was performed at baseline (on day 5 of the cycle) and repeated after 6 weeks of naltrexone administration. Plasma glucose, insulin and connecting peptide (c-peptide) concentrations were evaluated in all samples. Based on their insulinaemic response to OGTT, patients were classified as hyperinsulinaemic or normoinsulinaemic. Naltrexone treatment significantly (P < 0.007) reduced the insulin response to OGTT in the hyperinsulinaemic group without affecting the c-peptide incremental area; in the normoinsulinaemic group there was a slight, but not significant, increase in both c-peptide and insulin incremental areas. The two groups showed similar c-peptide incremental areas after naltrexone treatment. There was no significant difference in the c-peptide:insulin incremental areas molar ratio between the two groups; after treatment, a significant increase in this ratio was observed in both groups. When we considered the data as an expression of the fractional hepatic extraction of insulin, we found a lower value for hyperinsulinaemic in comparison with normoinsulinaemic patients (not significant), and a significant (P