Quantitative changes in testes of roe deer were studied during the annual cycle. Testicular spermatozoa were counted and proportions of different cell types were estimated using DNA flow cytometry. A proliferation-specific antigen of somatic cells was evaluated by an immunoradiometric assay. Apoptosis was examined by cell death detection ELISA, and testosterone concentrations were measured with an enzymeimmunoassay. The testis mass of adults reached a maximum during the rut from mid-July to mid-August. Gonadal size corresponded to numbers of testicular spermatozoa g-1 testis. In the rutting period, epididymal spermatozoa were of the highest morphological and functional competence. The proportions of haploid (1c), diploid (2c) and tetraploid (4c) cells changed over time with the maximum of 1c cells during the breeding period. Meiotic division (1c:4c ratio) increased sharply immediately before rut, while mitosis (% cells in G2-M phase) was already high during spring. Proliferation and apoptosis revealed an opposite pattern during the annual cycle; the most intensive apoptosis occurred during the time of testis involution. Testosterone production showed a biphasic pattern. It dropped rapidly from the highest value in August to very low concentrations thereafter. Yearlings were characterized by smaller peaks of testicular growth and sperm production. Fawns started testicular growth and meiosis in winter. In conclusion, the production of spermatozoa in roe deer is intensified by enlargement of gonads as well as enhanced efficiency of spermatogenesis during the rut. Interrupted proliferation and stimulated apoptosis promote testis involution after the rut, and testosterone seems to play a role in the regulation of both processes.