Babies show individual behavioral characteristics that distinguish them from one another even at birth; the authors devised techniques to measure these differences in order to determine whether or not basic temperamental differences are important in evaluating the results of various ways of rearing children. Change in the heart rate in response to various stimuli was studied; this change proved to vary inversely with the degree of the heart rate response of the babies at the time of stimulation. The results suggest a neonatal barrier against stimuli. Comparative evaluations of differences in the responses of individuals appear to be meaningful only in relation to certain specific relationships within each individual infant which must be determined prior to the comparative study.