Tissue wet weight, nucleic acid content and epithelial and stromal cell numbers were measured in the seminal vesicles of sexually mature male rats. After castration, tissue weight and RNA decreased rapidly and in aprallel to reach, after 14 days, values only 15-20% of those in control (not castrated) animals. During this period, DNA decreased to a much lesser extent (by about 40%), but this change in DNA correlates well with the observed loss of cells from the epithelium. Testosterone in vivo promoted an immediate resynthesis of RNA, the value characteristic of control animals being reached within 80h. Delays occurred in the hormone-induced regain of tissue weight (30h) and DNA (40h), each of which preceded proliferation of the epithelium (40--50h). The cells of the stroma were unaffected by these changes in the androgenic statls of the animal. It is suggested that these proliferative changes in the epithelium cannot account for the previously reported induction by testosterone of basic secretory proteins in this tissue.