The production of prolactin by explants of late secretory endometrium has been correlated with the extent of decidual differentiation. This correlation is strengthened by the observation that luteal phase defective endometrium produces less prolactin than normal control endometrium in a 24-hour in vitro culture system. In the present study the prolactin production by explants of normal, luteal phase defective, progesterone-corrected luteal phase defective, and clomiphene- or follicle-stimulating hormone/luteinizing hormone-corrected luteal phase defective late secretory endometrium was measured over 96 hours at 24-hour intervals. Progesterone in physiologic concentrations was added to the culture medium to maintain tissue integrity and prolactin synthesis. The prolactin production of normal late secretory endometrium rose over 96 hours under progesterone stimulation. The luteal phase defective endometrium produced significantly less prolactin under the same conditions. Histologically proven corrected luteal phase defective endometrium, regardless of treatment method, produced prolactin not different from the normal controls of the same dates. From these results it is concluded that histologic correction of luteal phase defective endometrium is associated with a corresponding biochemical correction with use of prolactin as a metabolic marker. The findings also strongly support timed endometrial biopsy as the method of diagnosis and evaluation of treatment of luteal phase defect.