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Cardiac rate was recorded by lightweight leads from unrestrained pups under 4 conditions: in the home cage litter situation, while being picked up by the experimenter, during 6 min alone in a novel environment, and in response to a startle stimulus. Resting heart rates in the litter showed evidence of high sympathetic activity during the second week of life; cardiac responses to stimulation were consistently deceleratory. By 20 days, the adult pattern of low resting rates and tachycardia upon stimulation had become established. Patterned changes in cardiac rate accompanied behavioral rapid eye movement sleep in younger pups and self-grooming in older pups. These distinct stages in the development of autonomic cardiac control may indicate critical periods within which early experience can differentially shape the pattern of adult cardiac responses.