The aim of this work was 10 document the validity and usefulness of progesterone determination instead of endometrial biopsies as evidence for ovulation within a menstrual cycle. The presence of progesterone in amounts greater than 2 ng/ml of plasma was always associated with a secretory endometrium, ie, the endometrium was matured beyond cycle day M as diagnosed by classical histologic dating. The remarkable simplicity of quantitating Inteal phase progesterone levels by this radioimmunoassay warrants their extended use for evaluating the physiology and the pathology of the corpus luteum.