In December and January the frequency of heart beat in Limax exhibits [mu] = 11,500 [plus or minus] 250. The ingestion of a small volume of sugar solution results in temporary change of [mu] to 16,200[plus or minus]320, which accords quantitatively with the value obtained from these slugs in spring. This effect of the sugar is reversible, but lasts longer than the abolition of negative phototro-pism which the sugar also produces. Other instances are given in which the value of the temperature characteristics for vital processes have been changed experimentally. The new values which appear have already been obtained in connection with homologous activities. These results confirm the view that the critical thermal increments serve to characterize recognizably different governing reactions in living matter, and indicate a basis for specific experimental control.