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The neonatal heart rate (HR) response to brief stimuli has generally been reported to be a monophasic acceleration. In this study brief auditory clicks to 6 neonates elicited, to a significantly greater degree than in non-stimulus conditions: 1) deceleration alone, 2) acceleration alone, and 3) a biphasic response consisting of an initial deceleration and a subsequent acceleration. The data are not yet available which would permit selection among the orienting reflex (OR), defense reflex, startle reflex, and Moro reflex models of these HR responses. With regard to peripheral mechanism, the early decelerative response was probably mediated by increased vagal inhibitory tone. The later accelerative response may have been mediated by one or more of the following mechanisms: decreased vagal inhibitory tone, activation of vagal accelerator fibers, and/or activation of sympathetic accelerator fibers. [ABSTRACT FROM AUTHOR]