An evident degranulation of metachromatic granules from mesenteric tissue mast cells was observed in more than half of experimental rats which were intraperitoneally given 10cc of stearic monoglyceride suspension in warm Tyrode solution (5Omg. of stearic monoglyceride in 10cc of Tyrode solution). A fairly light degranulation of metachro-matic granules from mesenteric mast cells was also displayed by the rats fed ad libitum with butter for 6 hours after being deprived of food for 24 hours.