Idiopathic PRL deficiency was detected in a parturient woman with otherwise normal pituitary function. This PRL disorder first became manifest postpartum when she had no milk production, and oral metoclopramide failed to raise her serum PRL levels. Her second pregnancy occurred spontaneously after 3 yr of attempts to conceive. During the pregnancy, her serum PRL concentration was very low, varying from 4.5-7.8 ng/ml, and the puerperium was again characterized by alactogenesis. During normal menstrual cycles and after iv GnRH, TRH, metoclopramide, and insulin tolerance tests, serum PRL was only rarely detectable by RIA, at very low concentrations. Bioassay results confirmed the PRL deficiency. The results confirm that PRL is necessary for puerperal lactation and suggest that it is needed for normal ovarian function. The present data also suggest that the maternal pituitary is the main source of serum PRL during pregnancy, and the decidua has only a minor contribution in this respect.