Ten patients suffering from endometriosis were treated with danazol at a dose of 200 mg three times daily for 6 months. To evaluate prolactin secretion, a thyrotropin releasing hormone (TRH) test and an insulin challenge test were carried out in the follicular phase before treatment, during the 6th month of treatment and after the reappearance of the second menses. The same tests were also performed, only once, in the follicular phase of ten normal women. The absence of a significant difference in basal prolactin levels as well as the response to the TRH and insulin challenge tests between controls and patients with endometriosis, before and after danazol treatment, support the view that hyperprolactinemia should not be considered as a cause of infertility in endometriotic women. Furthermore, the significant reduction of prolactin response to provocative tests during danazol treatment may be associated with the low estrogen levels caused by the medication. Finally, the inadequate prolactin response to insulin-induced hypoglycemia in patients with endometriosis and in normal women shows that this provocative test is not so effective for hypothalamic stimulation of prolactin.