Testis weights on 315 European brown hare from different ages were measured during the whole year around in Bulgaria and testicular activity was studied. From the results, we determinate, that the active reproductive season finish at end of August and beginning of September, whereupon follow period of calm when the testis weights were lowest. In September, October, and the beginning of November, the weights of the testes were low. The first indications for starting of the new reproductive season were shown in November, gradually with growth of the testis weights. In January the testis weights reached high levels, as the highest weights were measured in May-June and with a bit lower values in July and August. After this follow low weights, when the calm period started in September and the testis growth is reactive in December. All adult males (above 1 year old, determined by dry eye lens weights, n = 160), had high significant values (p< 0.0001) of testis weights during the whole year, compare with all young ones (under 1 year old, determined by dry eye lens weights, n = 155). The results showed that normal reproductive cycle with periods of activity and calm in male European brown hares from Bulgaria. The distribution of testes weights, compared to the various ages, follows the same sequence, as at the sperm density (we assigned sperm quantity for each individual in the epidydimides to five classes: 1 – no sperm, 2 – very little, 3 – moderately present, 4 – high amount, 5 – massive amount). View of the fact that the density of sperm is defined by the tail of the epididymis, and the weights were measured from the testes, the connection between them confirms the proofs about the existence of a normal reproductive cycle with stages of activity and standstill. The heaviest testes had the biggest quantity of sperm in the epididymis, independently from the season. We defined, that the quantity of sperm and the weights of the testes were in direct dependency (p < 0.0001), as exactly following the line of activity and standstill in the seasons.
