To show the progress made in acquiring knowledge of the relation of climate to the physiology of the dairy cow, the results of a comparative analysis of the reactions to different temperatures of three categories of cattle are presented. The studies dealt with cattle of the European, Indian, and Santa Gertrudis breeds housed in climatic chambers of constant day and night temperature with the same wall, ceiling, and air temperatures and the same humidity and air movement. Under these conditions, the “comfort zone” of European cattle was found to be between about 30° and 60° F. and that of Indian cattle 50° to 80° F. The higher heat tolerance of the Indian cattle seemed to be due to lower heat production, greater surface area per unit weight, shorter hair, and other body-temperature regulating mechanisms not visually apparent. The larger the animal and the higher her productive level, the lower was the “comfort zone” temperature. The inner body temperature was shown to be constant at about 20° F. above the comfort zone by an S-shaped increase in evaporative cooling. At 105° F. environment the near-lethal temperature of 108° F. was reached in the Holsteins, 106° F. in the Jerseys, and 105° F. in the Indian cows of the same weight as the Jerseys. At environmental temperature of 105° to 107° F. the European cows were near collapse. The Santa Gertrudis were found to be near Brahmans in heat tolerance; they grew equally well and maintained the same rectal temperature at 80° F. as at 50° F. The Santa Gertrudis were more cold-tolerant than the Indian heifers. Their critical temperatures have not yet been determined. The relative cold tolerance of Holstein and Zebu is indicated by a 2% increase in heat production in Holsteins and 60% increase in Zebu on lowering the temperature from 50° to 7° F. Acclimatized European cattle are cold tolerant and are not in need of protection against cold as such but only against wind, snow, and rain. They do need protection against heat. These findings show the practicability of loose-housing methods that have evolved in recent years and are now being commonly used by northern dairymen.