Serum prolactin (PRL) levels were determined by radioimmunoassay in 12 normal, menstruating women and in 26 infetile women with ovarian dysfunction for one cycle or for about 30 days in amenorrheic women. Galactorrhea was not observed. No significant change in serum PRL levels was observed throughout the normal, menstrual cycle. Of the 26 patients with ovarian dysfunction, seven showed PRL levels higher than normal. Twelve of the 26 patients were treated with clomiphene citrate (Clomid), 100 mg, daily for 5 days. Ovulation occurred in seven, and pregnancy was achi-ved in two of them. Serum PR;, follicle-stimulating hormone, and luteinizing hormone were measured for a preceding control cycle and during the cycle following Clomid treatment in the 12 patients. The serum PRL levels were normal during the control cycle in five of the patients who ovulated with Clomid and high in four patients who failed to ovulate. Although serum PRL levels were not significantly changed by Clomid in the patients who ovulated with the drug, they were markedly decreased during and immediately after Clomid treatment in patients who failed to ovulate with Clomid.