The in vitro diffusion of progesterone from a gel formulation using the European Pharmacopoeia method for transdermal dosage forms is described. The membranes used were the dorsal and ventral portions of the shed skin of three different species of snake. Considerable differences are apparent between the dorsal and ventral sites and between the different species of snake. The dorsal area shows better permeability for progesterone and the permeability order for the different species is python>cobra>viper. These differences may be due to the thickness of the skin and the hinge:scale ratio. The results indicate that shed snake skin is not a model membrane for human skin