The weight and histology of the testis and plasma LH levels were analysed after hemicastration of adult Ile-de-France rams in spring or in autumn. After hemicastration, the remaining testes were significantly heavier than those of entire animals measured at the same time of year. At 4 or 6 months after hemicastration performed in spring, the remaining testes were hypertrophied by nearly 40% as compared to the testes of entire sexually active animals, assessed in autumn. The variations of intertubular tissue volume, total seminiferous tubule length, stem cell stocks, daily production of round spermatids, and cellular volume of primary spermatocytes paralleled the variations in testis weight. The annual decrease of the area of the Sertoli cell nuclei and of the yield of meiosis and beginning of spermiogenesis during the non-breeding season was prevented by hemicastration performed in autumn. Plasma LH levels were consistently elevated till autumn after hemicastration performed in spring. A positive and significant correlation was observed between LH levels and yields of spermatogonial divisions.