University Department of Steroid Biochemistry, Royal Infirtnary, Glasgow, G4 OSF (Received 18 October 1974) It is generally assumed that only free steroid is removed from blood during passage through the tissue capillaries, as the rate of transfer of plasma protein, and therefore of protein-steroid complexes, into the extravascular space is very small in most organs (Dewey, 1959). However, it is possible that in target tissues with high steroid uptake, in addition to testosterone originally free in blood, some steroid dissociating from the complex can also diffuse into the cells. To test this hypothesis, a fixed number of slices from human hyperplastic prostate were disposed in one layer and sealed between two sheets of a membrane impermeable to protein, but permeable to steroids (Cuprophane PT 150, British Cellophane Ltd, Bridgwater). The various media shown in Table 1 were equilibrated for 40 min at room temperature with [3H] testosterone and superfused on