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The insulin-like growth factor-I (IGF-I) is a key regulator of reproductive functions. IGF-I actions are primarily mediated by IGF-IR. The main objective of this research was to evaluate the presence of IGF-I and IGF-I Receptor (IGF-IR) in stallion testicular tissue. The hypotheses of this study were (i) IGF-I and IGF-IR are present in stallion testicular cells including Leydig, Sertoli, and developing germ cells, and (ii) the immunolabelling of IGF-I and IGF-IR varies with age. Testicular tissues from groups of 4 stallions in different developmental ages were used. Rabbit anti-human polyclonal antibodies against IGF-I and IGF-IR were used as primary antibodies for immunohistochemistry and Western blot. At the pre-pubertal and pubertal stages, IGF-I immunolabelling was present in spermatogonia and Leydig cells. At post-pubertal, adult and aged stages, immunolabelling of IGF-I was observed in spermatogenic cells (spermatogonia, spermatocyte, spermatid, and spermatozoa) and Leydig cells. Immunolabelling of IGF-IR was observed in spermatogonia and Leydig cells at the pre-pubertal stage. The immunolabelling becomes stronger as the age of animals advance through the post-pubertal stage. Strong immunolabelling of IGF-IR was observed in spermatogonia and Leydig cells at post-puberty, adult and aged stallions; and faint labelling was seen in spermatocytes at these ages. Immunolabelling of IGF-I and IGF-IR was not observed in Sertoli cells. In conclusion, IGF-I is localized in equine spermatogenic and Leydig cells, and IGF-IR is present in spermatogonia, spermatocytes and Leydig cells, suggesting that the IGF-I may be involved in equine spermatogenesis and Leydig cell function as a paracrine/autocrine factor.