The effect of cyproterone acetate (CA) on the pituitary-testicular axis was studied in 6 healthy men. A dose of 300 mg of CA was administered orally in the early morning, after 3 h blood samples were collected using a multiple sample technique. Testosterone (T) levels were decreased by CA in all subjects (p 0.01), LH in all (p < 0.01) but one whereas 17-hydroxyprogesterone did not show any significant variation. In vitro, using an equilibrium dialysis, CA displaced T from plasma-binding proteins in males at 37 °C. The role of testosterone-binding globulin on the effects of CA on the pituitary-testicular axis remains to be clarified.