ABSTRACTHeart rate (HR) response to simple stimuli apparently changes from an accelerative to decelerative response in the first few months of life. The present study of 4 month old infants showed that response depends on state and on the intensity and rise time of stimuli. The response was solely decelerative in alert infants but in less alert states of increasing agitation or increasing somnolence, deceleration lessened and acceleration appeared. Rapid rise time had an acceleratory effect which was more pronounced during sleep than during the waking state. Results were compared with those obtained to the same stimuli in parallel studies of newborns and adults. It appears that the decelerative response in awakeSs, presumably an orienting response, has a curvilinear relationship to age which cannot be ascribed to differences in initial HR level, state, stimulus intensity, or stimulus rise time.