This report represents a preliminary attempt to define the best avenues of approach for selecting animals adapted to a tropical environment by using a climatic chamber as a means of testing their adaptability.Several studies have been recently published that describe differences in the reactions of animals of various breeds as they are subjected to high ambient temperatures (Kibler and Brody, 1951; McDowell et al.,1954). In all these studies, however, the animals tested were born and reared in temperate climates, so that they may not have been physiologically adjusted to the new environment to the fullest extent possible. All animals used in the present study were born at or near Turrialba, and had always lived in ambient temperatures of about 20° C., and in the high humidity characteristic of a rain forest type of tropical climate. Furthermore, this study presents some new and significant comparisons between the behavior of animals toward tests conducted at night and in the daytime.