The effects of castration on the synthesis (accumulation) of a major seminal vesicle secretory protein (SVS IV) were examined in young adult rats. In vitro incorporation of labeled amino acids into SVS IV by minced tissue was monitored by immunological methods. Castration resulted in a large decrease in the differential synthesis of SVS IV. A significant decrease in the relative incorporation of isotope into SVS IV was evident within 3 days of castration, and by 4 weeks relative incorporation dropped some 30-fold. These changes took place in the presence of a large generalized decline in protein synthesis so that incorporation into SVS IV on an organ basis decreased by over 200-fold. SVS IV messenger RNA levels were estimated by RNA excess solution hybridization using a cloned cDNA probe. Relative message levels declined after castration in harmony with the declines in SVS IV synthesis. SVS IV mRNA was decreased by a relative factor of approximately 20 and an absolute factor of approximately 200 in long-term (40-day) castrates. Accordingly, the seminal vesicle conforms to the general pattern of steroid regulated systems in which hormone withdrawal leads to differential decreases in the steady-state pool size for specific mRNAs. The seminal vesicle is unusual, however, in that a prolonged period is required for maximum differential effects to occur.