Activin A, encoded by Inhba, and activin B, encoded by Inhbb, regulate epididymal development and inflammatory responses. Their expression is highest in the caput epididymis and lowest in the cauda. Activin A and B have been localised by immunohistochemistry primarily to the epithelial principal cells and interstitial macrophages. Efferent duct ligation had no significant effect on Inhba expression, but Inhbb was reduced by more than 50%, indicating that activin B, but not activin A, is regulated by lumicrine factors from the testis. In order to clarify these differential responses, the precise cellular sources of activins were investigated in 25 and 56 day old C57/Bl6 mice using in-situ hybridisation (ISH), and RT-qPCR analysis of the proximal (segments 1-3), and distal caput (segments 4-5) epididymal fragments. Both activins were more abundantly expressed in the proximal caput than the distal caput. Proximal Inhba levels were about twice the levels in the distal caput, while Inhbb was almost undetectable in the distal caput. Unexpectedly, Inhba was localised by ISH to peritubular and interstitial immune cells, with minimal expression by principal cells. Highest expression was in the efferent ducts and segment 2. Inhbb expression was localized to epithelial, especially principal, cells, and was mostly confined to segment 1 and the efferent ducts. The expression pattern was the same at both ages. These data indicate that activin A is produced mostly by peri-epithelial and interstitial cells, while activin B is produced predominantly in the epithelium. Accordingly, lumicrine factors regulate Inhbb in segment 1, but have marginal effects on Inhba, which is expressed more widely in the caput. These largely segregated production sites suggest slightly different roles in epididymal function. Activin A origins in the interstitium may signify a more important role in immunoregulation, while activin B may be more involved in epithelial function and sperm maturation.