The levels of prostaglandins in the human endometrium throughout the normal menstrual cycle were measured by bioassay. The identity of prostaglandin F(2alpha) and prostaglandin E(2) in endometrial tissue extracts was established by combined gas chromatography/mass spectrometry.The levels of prostaglandin F(2alpha) and prostaglandin E(2) are low during the proliferative phase of the cycle (10-25 ng/100 mg tissue). Prostaglandin F(2alpha) rises significantly during the luteal phase to levels of 65-75 ng/100 mg of tissue. Prostaglandin E(2) levels are similar to prostaglandin F(2alpha) levels in the proliferative phase but remain lower during the luteal phase. The prostaglandin E(2) level is highest at menstruation (52 ng/100 mg tissue). The implications of these findings are discussed in relation to the menstrual process and to the steroid hormonal changes during the cycle.