The epididymis of stallion castrated during the breeding and non breeding seasons were subdivided into six regions and their ultrastructural and cytochemical characteristics were studied in order to provide a better understanding of the structure-function relationship of this androgen target organ. Even when the stallion has been postulated to be a seasonal breeder, our results do not show significant ultrastructural or cytochemical differences in both seasons. The pseudostratified epithelium is composed mainly of principal and basal cells and intraepithelial lymphocytes. The principal cells show morphological features of protein or glycoprotein secretion, especially in the caput epididymidis. Although PAS, CFH and AB positive substances were found throughout the epididymis, the reactivity was maximal in the caput region. This positive reaction can be ascribed to acidic glycoproteins. In stallion tissues, 4.0 acetylated sialic acid occurs in relatively high amount and is possible that the acid glycoprotein observed in our material have also this characteristic. The principal cells of the distal caput and corpus epididymides also display morphological hallmarks of absorptive and anabolic activity. These results are consistent with the histological reactions that demonstrate that the enzymes involved in active transport showed the strongest reaction in the corpus region. The acid phosphatase reaction was also strongest in these segments. In the cauda region, where the spermatozoa are stored ready for ejaculation, morphological signs of metabolic activity were also observed, but less notorious than in the more proximal segments. Resorption of non ejaculated spermatozoa was also observed in this region. It is difficult to evaluate the functional meaning of the spermatophagy in the epididymis because the images of sperm phagocytosed by epithelial cells were seen only in one or two cases. The chemical composition of the epididymal fluids changes along the length of this organ, concomitantly with the sperm maturation process, and it is possible to assume that some of these changes are a result of the secretory and absorptive activities of the principal cells.