Heart rate in the newborn and very young rat was recorded by using a Davis electroencephalograph. Results indicate an atypical developmental sequence. The avg. rate at birth of 131 animals was approx. 171/min. This rate is significantly lower than the avg. adult rate of approx. 450. Within the first 24 hrs. of life significant increases in heart rate occur and by the 10th day of life the avg. heart rate is approx. 400/min. Sex differences in heart rate, found in tamed mature animals, were not present at birth but became manifest after the 10th day.