Testicular activity in brown hare Lepus europaeus Pallas, 1778 was studied during the annual cycle. Testicular mass, spermatozoa and testosterone were estimated. Percentages of haploid, diploid and tetraploid cells were monitored using DNA flow cytometry and the proportions of somatic and spermatogenetic cells were determined after selective labelling of somatic cells. Testis mass was high from January to July and declined thereafter to the nadir in September. Testis growth was reactivated significantly in December. Changes in testis mass corresponded with the spermatogenic efficacy (spermatozoa/g testis). High spermatogenic activity was characterized by intensive meiotic transformation of spermatocytes to spermatids, high percentages of haploid cells and low proportions of cells in the G2/M phase of mitosis. Proportions of haploid cells declined rapidly during the testis involution. Spermatogenesis was newly activated from November. The proportions as well as the ratios of spermatogenic and somatic cells differed significantly between the periods of testis involution and recrudescence, Testosterone level showed a pronounced increase in autumn preceding the intensification of spermatogenesis; the lowest concentration was found during prominent testis involution in August. The results suggest that the regulation of seasonal testicular activity is characterized by co-ordinated shifts in the relationships between mitosis, meiosis and testosterone production.
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