1. Changes in the histological structure and biochemical composition have been studied in the testes and epididymis of dog after unilateral and bilateral vasectomies. 2. Testes and epididymis weight did not change after 30 days of unilateral vasectomy. Whereas a significant decrease was observed after 60 and 120 days of unilateral and bilateral vasectomy. Tests and epididymis weights returned to normal after 180 days of bilateral vasectomy. 3. Histologically testes and epididymis were normal after 30 days of vasectomy. After 60 and 120 days of unilateral and bilateral vasectomy. Testes and generative changes. Seminiferous tubule and Leydig cell nuclear diameter reduced. Sixty to seventy percent tubules recover after 180 days of bilateral vasectomy. 4. Decrease in RNA, protein, sialic acid contents and increase in alkaline phosphatase activity, total lipids, cholesterol coincided with the absence of spermatozoa and atrophic changes in the testes. 5. The early degenerative changes were caused due to back pressure of testicular fluid (Vare et al, 1973).