Abstract Fourteen four-month-old infants were presented with a tone of moderate intensity, similar to one previously used with newborns, to determine if cardiac response patterns were the same in the two age groups. Stimulus repetition effects and then dishabituation to a nonfamiliar stimulus were also investigated. While the newborns had responded to this auditory stimulus with cardiac acceleration, the older infants showed deceleration. The largest response decrement occurred between the first six trials and the remaining trials. Response to the first nonfamiliar stimulus differed from response on the immediately preceding trial. It was hypothesized that the appearance of a decelerative heart-rate response in the early months of life may signify the development of the orienting response.