Functional development of the ventral prostate and seminal vesicles in the male rat has been examined by incubating tissue with [35S] methionine and using specific antisera to detect the synthesis of individual marker proteins. 4 major secretory proteins of the seminal vesicles were chosen along with 4 major prostatic secretory peptides, including 3 which form the prostatic steroid-binding protein. None of the seminal vesicle proteins was synthesised before 25-30 days but all were then induced in parallel to reach adult values by 45-50 days. The 3 peptides of the prostatic steroid-binding protein were synthesised at the earliest age studied (10 days) and were also induced in parallel to maximal levels by 25-30 days. In contrast, the 4th prostatic marker, a glycopeptide, was not synthesised until about 25 days. Treatment of male rats from 3 days of age with testosterone induced precocious development of the seminal vesicles, causing substantial synthesis of marker proteins well before their synthesis would normally have been expected, and also advanced the synthesis of the prostatic 22 K glycopeptide. The results confirm that functional development in the ventral prostate precedes that of the seminal vesicles but that within the ventral prostate there is a considerable difference in the developmental profiles of its secretory proteins.