The effects of castration on the rat ventral prostate was studied utilizing an improved histoquantitative technique. Both volumetric fractions of the tissue compartments and their fractional weights, or, 'total amounts', were calculated during an observation period of 30 days. In addition, the surface area, length and mean diameter of the glandular tubules were recorded. The changes in the mean free distance between the tubules were recorded. The changes in the mean free distance between the tubules ('thickness' of the interacinar tissue) and the mean distance between the glandular centres were also determined. It was observed that the prostatic epithelium was quickly reduced in thickness after castration, to one half at day 2. Decline of the fractional amount of the lumen was slower; it also reached below the 10% level at day 30. The amount of interacinar tissue first increased at 12 hours by one third, but from then on decreased to one third of the normal amount. The 'thickness' of the stroma almost doubled, which was probably due to the sum of the simultaneous marked decline in the diameter of the tubules to one third of the original and the less striking reduction to two thirds of the mean distance between the glandular centres. The morphometrical method ensured the acquisition of a quantitative insight into the tissue processes involved in prostatic atrophy. Calculation of the fractional weights was regarded as especially invaluable, inasmuch as a growing body of evidence has been accumulated in favour of the crucial role of stromal-epithelial interactions in the differentiation, and growth, of the prostate.