Ten mongrel adult dogs have been studied in 140 attacks of blacktongue over a period of 2 years. Test doses of nicotinic acid and nicotinic acid amide have been given for periods of 10 days to dogs subsisting on a blacktongue producing diet of natural foodstuffs and the period of protection against subsequent attacks observed at the following daily dose levels: 0.1 mg., 0.2 mg., 0.3 mg., 0.4 mg., 0.5 mg., 1.0 mg., 5.0 mg., 10 mg. and 20 mg. per kilo of body weight. An increased protection was observed with increase in dose level. The increase in protection was not, however, proportional to the increase in dose level but was a function of the logarithm of the dose. These results indicate a limited, but definite storage of nicotinic acid, or one of its physiological derivatives in the body of the dog.