Present studies deal with the role of inhibin in proliferation and growth. The effect of inhibin on incorporation of 3H-thymidine in prostatic DNA in vivo as well as by NRK-49F and Balb/c3T3 cell lines in vitro, was investigated. Also studied the immunocytochemical localization of inhibin in normally proliferating and differentiated tissues of human prostate and endometrium. The in vivo studies revealed a suppression of 3H-thymidine uptake both in ventral (33%) and dorsolateral (26%) lobes of rat prostate. Interestingly, the histology of inhibin treated rat prostate manifested amidst the epithelial lining, an appearance of apoptotic bodies which are considered to be indicative of cell death. Further, the immunocytochemical studies for localization of inhibin showed intense staining in the differentiated human prostate and endometrium as compared to the respective proliferative tissues. Is inhibin kept suppressed in these proliferating tissues, because it is antiproliferative? The present in vitro experiments demonstrated that, at low inhibin concentrations, the incorporation of 3H-thymidine is stimulated while at higher doses it is suppressed. Thus, it is clear that prostatic inhibin seems to have a concentration-dependent dual role in the regulation of DNA synthesis.