Abstract 1. 1. Male specimens of the genera Ateles ater and Cebus apella were used to study the effect of hypothermia on the heart. The heart was monitored using a direct-writing electrocardiograph. Needle electrodes were devised to fit the small limbs of the animals. Recordings were made in restrained and unrestrained animals at body temperatures which varied from 35°C down to 25°C. 2. 2. The results indicated that heart rate varies inversely with body temperature. There were some differences between the two species, but in general the responses were similar. 3. 3. The relationship of the QT interval to the total cycle length was calculated using the square root formula. When the logarithm of K was plotted against the logarithm of the heart rate a straight line was obtained showing that K decreases with higher heart rates. The slope for Cebus data is steeper than for Ateles. 4. 4. There was virtually a linear prolongation of the T wave as body temperature and heart rate decreased. The so-called Q 10 law receives little support from the data.