The variations in the appearance and number of the tongue tissue mast cell of the healthy rats weighing 130-150 gm were observed in 20 microscope fields under the magnification of 430x after placing various kinds of taste stimulating reagents such as sugar, quinine, salt, tartaric acid and hot pepper powder on the tongue mucous membrane, and the following conclusions are obtained. 1) The total number of mast cells seen on the longitudinal tongue section of healthy rats averaged at 121.0±15.03 with normal mast cells comprising 93.2% and the moderately and highly diffuse mast cells comprising 5.7% and 0.9% respectively. 2) With the placement of various taste stimulating reagents, the number of mast cells seen showed decrease at 10 minutes thereafter, reaching lowest values at 30 minutes after the placement. Gradual increase was observed thereafter reaching the normal range in 5 to 7 hours after the placement. 3) The influence of various taste stimulating reagents on the tongue tissue mast cells was variable in degree, showing stronger effect in the order of sugar, quinine, salt, tartaric acid and the hot pepper powder ; the former three showing changes not so marked, but the tartaric acid showed moderate changes in the number and appearance of the mast cells and the hot pepper powder showing marked influences definitely. 4) Though the influences of sugar, quinine, and salt on the tongue tissue mast cells were mild, the temporal changes in the appearance and number of the mast cells were similar to that seen in the experiments with tartaric acid and hot pepper powder, suggesting that the influence of these taste stimulating reagents on the tongue tissue mast cells are the same with some quantitative difference. 5) In general the ratio of normal mast cells were lowest when the total number of mast cells were the lowest, i.e., 30 minutes after the placement of reagents, with the ratio of diffuse mast cells at its highest. Thus the normal mast cells showed increase and the diffuse mast cells decreased with the increase of total number of mast cells. 6) From the above experimental results, it is seen that the palcement of various taste stimulating reagents on the tongue mucous membrane will cause the decrease of total number of tongue tissue mast cells and the increase in the diffused mast cells upto 30 minutes thereafter, with gradual recovery to normal state in 5 to 7 hours.