A Progesterone receptor (PR) has been detected in the reproductive system of both vertebrates and invertebrates and in the liver of vertebrates. In some of these groups PR resembles the PR structure of mammals and birds, existing with two subunits PR-A and PR-B. On the contrary, in other groups PR is present with only one subunit. When present the two subunits can be expressed in the same tissue or can be differentially expressed suggesting tissue-specific expression pattern, PR subunit(s) fluctuate throughout the reproductive cycle and may be differentially regulated by steroid hormones. Progesterone, is a versatile hormone exerting an ample variety of effects. Through the interaction with its specific receptor, progesterone seems to play a remarkably constant role on the reproductive system of the oviparous species of lower vertebrates, being responsible for the secretion of a complex proteinaceous gel-like material (albumen) and mucus, essential for lubrication of the passage of the eggs down the oviduct. A similar role has been also hypothesized for progesterone in the only invertebrate investigated so far, the cephalopod Octopus vulgaris. In vertebrates PR presence in the liver suggests its possible involvement as modulator of the vitellogenin synthesis during the ovarian cycle. Thus, PR may play a key role in transition from oviparity to viviparity by (a)regulating oviduct growth, function, and contractility and (b) suppressing vitellogenin production while favoring placentation. Since PR is immunologically similar in vertebrates and invertebrates, cross reacting with antibodies against PR-A and PR-B of birds, we hypothesize that PR can be regarded as a case of structural and functional parallelism.