Distribution of 3H-nicotine in 26 tissues of rhesus monkeys and dogs was studied following intravenous injection of 10, 100 and 500μg/kg of the drug. (1) It was confirmed in both species that within 5min. after injection the highest concentration of 3H-nicotine was observed in target organs such as central nervous system and adrenal gland. (2) High concentration of 3H-radioactivity 30min. after injection was found in the kidney, salivary gland, stomach and intestinal mucosa. These results suggest that excretion of nicotine from these organs begins soon after injection. (3) Distribution of 3H-nicotine in skeletal muscles and fatty tissues of monkeys was higher than those of dogs. This results in that distribution in central nervous system and serum of monkeys was lower than those of dogs. These differences may elucidate the factors of nicotine sensitivity among the species.