Abstract A method for the determination of progesterone concentration in human or rabbit myometrium is described. The tissue is first chemically digested in a mixture of NaOH and sodium dodecyl sulphate. After digestion which is relatively rapid in this mixture, progesterone was recovered completely by extraction with ethyl acetate. After a minor purification step (Sephadex LH-20 chromatography), progesterone was determined by radioimmunoassay. The method is capable of determining progesterone concentration in pregnant human myometrial tissues as small as 50 mg wet weight. The details of the method along with the data on its reliability are presented.