Administration of a melatonin-free pineal extract to hemicastrated rats in doses of 2 ml/day/animal induced a statistically significant decrease in their serum and testicular sialic acid after 3 days but not after 30 days of treatment. Unilaterally castrated rats receiving testosterone (6 injections) showed 10 days post-castration a statistically significant decrease in sialic acid in their serum and testes but not in the epididymis. Twenty-one days post-castration, time in which 11 testosterone injections were given, the sialic acid was low in the epididymis as well. Concomitant administration of testosterone and pineal extract caused a total reduction in the serum and partial reduction in the testes, of the lowering effect of testosterone on sialic acid. The return to the normal values of the serum sialic acid, to almost normal values of the testicular sialic acid and the increase over the normal values of the epididymal sialic acid shows the equilibrating role played by the pineal extract on the sialic acid in the hemicastrated rat treated with testosterone. The role of the melatonin-free pineal extract on sialic acid, a biochemical index involved in reproduction, is discussed.