The effects of testosterone (1 mg testosterone propionate/100 g body weight for 30 days) on the ventral prostates of rats castrated 30 days prior to the onset of hormone treatment was investigated with a stereologic morphometric method. Testosterone induced rapid growth of the prostate, which was reflected by a 29-fold increase in acinar parenchyma but only a 2.6-fold increase in interacinar tissue. There was a 23-fold increase in the total amount of glandular epithelium and a 36-fold increase in total luminal amount. Various volumetric fractions, total surface area, total tubule length, height of the epithelium, mean diameter of the tubules, and mean distance between the tubules and between the glandular centers were calculated. Growth of the acinar parenchyma, especially of the glandular lumen, was the dominant pattern. We could not demonstrate any remarkable responses in the interacinar tissue during any phase of testosterone treatment.