We are using specialized autoradiographic and microdissection techniques and other methodologies to study progesterone action in the amphibian oocyte, an immense cell in which steroids entrain striking physiological and biochemical changes. These studies include (1) the quantitative localization of progesterone receptors in the oocyte before and after progesterone treatment, using techniques that prevent artifactual redistribution; (2) the identification and quantitative assessment of the mechanisms responsible for the transmembrane and intracellular movement of progesterone; (3) the determination of the origin, chemical and physical nature, and cellular biochemical and biophysical steps that relate cell's membrane and protein synthesis responses to progesterone.