Annual changes in the concentration of serum testosterone (T) in sika deer stags were examined. The relationships between the serum T concentration and the size of testis, and between the serum T concentration and the antler cycle were also evaluated. The serum T concentration increased between July and September, then decreased between October and November. The highest serum T concentration was noted in September or October. During the period from November to the following July, the serum T

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... centration remained low. The volume of the testis and scrotal circumference showed changes similar to those in the serum T concentration. The testis volume showed clearer seasonal changes than those of the scrotal circumference. Shedding of velvets was observed during the period of high serum T concentrations. These results indicate that there are distinct annual changes in the blood T concentration in sika deer stags, which are related to the annual changes in testis volumes, scrotal circumferences and antlers.