Plasma prolactin concentrations were determined in 16 nonovulatory women during treatment with human meonpausal gonadotropins (hMG). In eight patients with initially normal prolactin levels of less than 20 ng. per milliliter, a significant rise was noted at the end of hMG administration, this is thought to be a response to increased endogenous estrogen concentrations. A similar rise in plasma prolactin levels was seen in some but not all of the eight patients with initially elevated "basal" prolactin concentrations. Three of these hyperprolactinemic patients had radiographic evidence of a pituitary lesion--either a pituitary adenoma or a "microadenoma"--but the variance in prolactin response could not be explained on this basis. The two groups of normo- and hyper-prolactinemic patients showed no significant difference in the required dosage and duration or hMG treatment, plasma estradiol-17 beta response, and ovulatory and pregnancy outcome.