During the 1990s, extensive research has effectively mapped the progesterone receptor-mediated actions of progesterone and has more recently uncovered nonreceptor-mediated effects--the effect of progesterone on uterine sensitivity to oxytocin, for example, involves direct, nongenomic progesterone action on the uterine oxytocin receptor. However, the majority of progesterone effects occur as a result of progesterone-receptor-mediated action, where progesterone behaves as a hormone-dependent transcription factor, probably because these receptors are widely distributed in the body. A distinguishing characteristic of progesterone receptors is the existence of two isoforms, A-form and B-form. In most tissues coexpressing progesterone receptors, estrogen controls the regulation of progesterone receptors, thereby also controlling sensitivity to progestins. Thus, progesterone receptor expression is upregulated by estrogen and downregulated by progesterone in most target tissues.