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Hypothalamic-pituitary-ovarian function, as well as other pituitary tropic hormone secretions, was examined in 27 hyperprolactinemic patients (17 with PRL-secreting tumor and 10 with so called functional hyperprolactinemia). Serum PRL levels ranged between 39 and 108 ng/ml in functional cases and between 34 and 2500 ng/ml in patients with pituitary adenoma. Basal serum LH, FSH and 17 β-estradiol levels did not differ from those recorded between days −14 and −10 in normally cycling women in functional cases, and were lowered in patients with pituitary tumor. LH response to GnRH (25μg iv) was normal in 7, impaired in 1 and exaggerated in 2 functional cases, and appeared normal in 9 and impaired in 8 tumoral cases. Two exaggerated FSH responses were seen in functional patients and 4 in patients with pituitary tumor. Estradiol benzoate administration (1 mg im) caused a significant fall in serum FSH and LH within 24 hr, but failed to induce serum LH increase within 96 hr in 11 out of 12 examined patients while a definite LH peak occurred within 48–72 hr in 7 patients retested after serum PRL normalization. Clomiphene citrate (100 mg/day for 5 days) administration caused a normal rise in serum LH, FSH and 17β-estradiol in 5 functional cases and in 1 case with pituitary tumor out of 9 patients tested. Normal pulsatility of serum LH was observed in 3 examined patients. In 3 normogonadotropic patients with PRL-secreting tumor, hCG + hMG administration (5000 + 150 IU im for 3 days) did not cause any significant increase in plasma 17β-estradiol while inducing a sharp rise in the same patients retested after serum PRL normalization. No abnormal changes in TSH, GH and ACTH secretion both in the basal state and after provocative stimuli were observed in patients with functional hyperprolactinemia, whereas in patients with pituitary tumors TSH secretion was impaired in 53% of cases, GH secretion in 71% and ACTH secretion in 29%. The present data along with previous reports in the literature suggest that high serum PRL can induce anovulation and amenorrhea by acting at both hypothalamic and ovarian levels.