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Summary.Reproductive organs and plasma testosterone levels were measured every 2 months from May 1975 to April 1977. Male wallabies became sexually mature at 18–20 months of age when the testes began to enlarge, the seminiferous tubules opened and increased in diameter, and spermatozoa first appeared. The weight of the testes and epididymides of sexually mature wallabies did not show any annual changes but there was a significant increase in the size of the prostate and Cowper's glands and in peripheral testosterone concentrations during the breeding season. These results support suggestions that the males respond directly to the breeding condition of the females.