Endometrial tissue from uteri of 35 nonpregnant, premenopausal women was assayed for prostaglandin E2 and F2 alpha binding site content as a function of the phase of the menstrual cycle and the pathologic state. For all specimens, tritium-labeled prostaglandin F2 alpha, binding was very low (less than 8 fmol/mg of protein) or undetectable regardless of the phase of the menstrual cycle or pathologic state or in the presence or absence of 10 mumol/L of indomethacin, a prostaglandin synthetase inhibitor. However, tritium-labeled prostaglandin E2 binding was detected in every specimen and was independent of the presence or absence of indomethacin. Binding of tritium-labeled prostaglandin E2, as determined by Scatchard analyses, was biphasic (dissociation constant approximately 1 nmol/L; dissociation constant for low-affinity sites approximately 10 nmol/L) for both proliferative and secretory endometrial tissue. However, the total number of prostaglandin E2 binding sites, determined from Scatchard or single-point analyses, was significantly higher (p less than 0.01) in proliferative endometrium compared to secretory endometrium. In addition, for endometrium from the proliferative phase of the menstrual cycle, the diagnosis of abnormal uterine bleeding was associated with higher (p less than 0.01) tritium-labeled prostaglandin E2 binding than diagnosis of dysmenorrhea, stress urinary incontinence and uterine prolapse, or pelvic inflammatory disease. Endometrial specimens with the last four diagnoses did not differ significantly (p greater than 0.1) from each other.