The effect of a single intratesticular injection of Freund's complete adjuvant (FCA), incomplete Freund's adjuvant (IFA) and various components has been investigated in dogs. FCA caused severe granulomatous reaction with widespread degeneration and vacuolation of the tubules. IFA and Arlacel A induced oligo - but not azoospermia, which was of short duration (6 weeks). Bacillus Calmette Guerin (BCG) suspended in saline given with uniform distribution at doses of 10 units or higher was effective in inducing aspermatogenesis. The sperm count declined to near zero level within 3 to 6 weeks after injection of BCG, and sperm suppression was maintained for 6 to 11 months of the observation period. At moderate doses of 10 to 25 units partial to complete inhibition of spermatogenesis was seen in different tubules with leukocyte infiltration in the interstitium. The basement membrane of the tubules, the peritubular cells as well as the Leydig cells were intact. At high doses (75–110 units) and at sites where bacillary deposits were high even in moderate dose animals, there was massive leukocyte infiltration with nonspecific damage to the tubules. The animals, on low or high dose, retained libido and copulated with bitches in oestrus. The mating was, however, infertile. The treatment, especially with low and moderate doses, was largely free of significant side effects. At higher doses, scrotal pain and swelling lasting for 7–15 days was observed; a small decrease in the size of the testes was also noted.