Despite clear indications of interleukin-1 (IL-1) action on Sertoli and germ cells, previous studies failed to detect IL-1 receptors (IL-1R) within the seminiferous tubules. Here, we investigated the existence of the type I signaling receptor (IL-1RI) and the type II decoy receptor (IL-1RII) mRNAs within the testis. Polymerase chain reaction analysis showed the presence of both receptor mRNAs in isolated rat, mouse, and human somatic testicular cells (macrophages, Leydig, Sertoli, and peritubular cells). While also present in rat and mouse isolated pachytene spermatocytes and early spermatids, these receptor mRNAs were not found in human germ cells. The distribution of both IL-1R mRNAs was then examined in adult rat and mouse testis using light and electron microscopic in situ hybridization. No IL-1RI signal was detected in rat testis. In mouse testis, we did not find any signal for IL-1RII. In contrast, IL-1RI mRNA was detected in a wide variety of mouse testicular cells. Strong expression was observed in the rete testis area and high expression was seen over the epithelium of the epididymal duct and in interstitial cells, while lower labeling was detected in peritubular and Sertoli cells and in all germ cell types from spermatogonia to early spermatids; no signal was seen in late spermatids. That the IL-IR was also strongly expressed in the interstitium, the rete testis and efferent duct areas, and the epididymis was established using an autoradiography technique. Overall, our study strongly supports the hypothesis that IL-1 is a regulator of testicular function of prime importance.