We studied PRL, FSH, and LH response to LRH in 82 anovulatory and 4 normally ovulating women. Ten anovulatory patients who were basally hyperprolactinemic showed no significant change in PRL concentration after LRH. Of the remaining 72 anovulatory patients with basal PRL levels in the normal range, 59 showed no PRL modification after LRH (as in normals) whereas in 13 patients, a prompt and significant rise of PRL concentration above basal levels in response to LRH was observed. In these 13 patients, the basal PRL levels were significantly higher than those of the other 59 normoprolactinemic women. No significant differences in gonadotropin concentrations were detected among the three groups. The unusual rise in PRL levels after LRH in these 13 patients can be interpreted as a paradoxical response of the pituitary to a specific stimulus, as seen in other clinical conditions. It is suggested that this phasic hyperprolactinemia might represent an intermediate phase between true normoprolactinemia and chronic hyperprolactinemia.