When heart rate is used as the index of conditioning, rat pups younger than 15 days of age do not display an odor-shock association. This constitutes a marked delay relative to the development of a somatomotor conditioned response. The incapacity to display autonomic learning to an olfactory stimulus prior to day 15 is not due to the inability to perceive and to orient to the olfactory stimuli used, nor to the inability to make unconditioned phasic cardiac changes. Rather, the late development of the heart rate conditioned response may indicate that the central neural mechanisms mediating heart rate conditioning are distinct from, and mature later than those mediating 1) heart rate orienting and 2) somatomotor conditioning. Evidence from studies in adult species is used to support these speculations.