It is well known that the frequency, severity and mortality of many diseases vary with different seasons of the year. That acute respiratory infections reach a peak in the late winter months, with a minor peak in the late autumn, is attributed to a lowered local resistance to infection with the unknown virus of 鈥渃ommon colds鈥� and the known pathogenic cocci, although as yet we have no satisfactory explanation as to why the exposure to cold lowers the resistance. To be sure, a physical chemist, Schade, would attribute to a decreased dispersion of the tissue colloids on cooling the increased vulnerability to bacteria, but this is pure hypothesis and does not explain how the invading bacteria fail to suffer a correspondingly reduced activity when chilled. In the lower animals, seasonal changes in metabolism are well recognized, such as the hibernation of some species, and the seasonal variation in reproductive activity, as well as the variations in growth of hair, feathers and antlers. Beckmann,¹ in a consideration of the effect of season on disease, attributes the absence of such marked seasonal alterations in man to the fact that with higher development comes a better regulating mechanism against extraneous influences. But even in man there are distinct alterations in metabolic activity at different times of the year. Thus, measurements have shown that in spring the hair grows more rapidly than at other times, while body activity as a whole is lessened in winter, so that the usual amount of time spent in sleep is much increased over the summer sleep among people not too artificially regulated by customs and alarm clocks. Presumably this is related to hibernation in other species. It is said that pulse rate, temperature and respiration are highest in winter, and recently it has been found that the height of the capacity of the blood to bind carbon dioxid is reached with the shortest days of the year. In the spring there is a distinct fall in the carbon dioxid tension of the blood, which implies a decrease in the alkali reserve.