This review discusses the biological roles of steroid sulfotransferase enzymes (ST's) and steroid sulfate sulfohydrolases (sulfatases) mainly in mammalian tissues. In addition, some consideration is given to certain characteristics of these enzymes and, where possible, to their biological control. A considerable number of ST's of varying specificities, substrate affinities, and kinetics appear to exist. Several of these possess the properties of regulatory enzymes. ST's which act upon estrogen in reproductive tissues, such as uterus, are of particularly high affinity, appear to be under some biological control, and may exert important effects upon estrogen action. Although biosynthetic pathways involving steroid sulfate intermediates have been described, their importance is difficult to determine. The presence of an esterified sulfate group on a steroid molecule may markedly affect the action of enzymes, such as hydroxylases, upon the steroid structure in both a qualitative and quantitative sense. The number of different steroid sulfatases is not well understood. A sterol sulfatase present in the female reproductive tract appears capable of destabilizing the sperm head membrane by hydrolyzing sterol sulfates necessary for its integrity, and hence enabling the fusion of sperm and ovum. Other sulfatases may utilize blood-borne steroid sulfates for the ultimate production of estrogen which, in fetal membranes, could play a role in parturition and, in breast tumours, could function as a growth promoting agent. Brain sulfatases could possibly produce steroid hormones for purposes of tissue differentiation and (or) feedback control mechanisms, but this is not firmly established.