AbstractBackground: Renin is classically secreted from juxtaglomerular cells of the kidney by endocrine or paracrine mechanisms. In a previous report, intense renin immunoreactivity was observed in the coagulating gland (CG), a new source for renin in mice. In the present study, immunoelectron microscopical analysis for renin was carried out to clarify the secretory site of CGs.Methods: Five adult male C57BL/6 mice were used in this study. The CGs were fixed with an ice‐cold 2% glutaraldehyde and 2% paraformaldehyde mixture and embedded in Lowicryl K4M. Ultrathin sections were treated with antimouse renin antiserum and colloidal gold (15 nm)‐labeled protein A complex.Results: In immunoelectron microscopical observation, renin was first detected at the Golgi vacuoles just budding from the lamellae, although it was not demonstrated in all Golgi vacuoles. In the production series of exocrine granules, renin immunoreactivity was observed in some granules that were distributed in the supranuclear region. Both renin‐positive and negative exocrine granules were secreted from the apical cell membrane by exocytosis. The lysosomal granules also showed stronger renin immunoreactivity and contained homogeneous or heterogeneous materials. In the supranuclear region, it was observed that exocrine granules were fused with irregular lysosomal granules. At the apical region, such lysosomal granules were closely associated with cell membrane. At the basolateral region, immunoreactivity for renin was localized in electron dense granules.Conclusions: These results suggest that part of the renin in the CGs is released by exocrine secretion into the genital tract. © 1995 Wiley‐Liss, Inc.