Previous research is inconsistent as to whether a more labile (faster-changing) autonomic system confers performance advantages, or disadvantages, in infants and children. To examine this, we presented a stimulus battery consisting of mixed static and dynamic viewing materials to a cohort of 63 typical 12-month-old infants. While viewing the battery, infants’ spontaneous visual attention (looks to and away from the screen) was measured. Concurrently, arousal was recorded via heart rate (HR), electrodermal activity, head velocity, and peripheral movement levels. In addition, stress reactivity was assessed using a mild behavioral stressor (watching a video of another infant crying). We found that infants who were generally more attentive showed smaller HR increases to the stressor. However, they also showed greater phasic autonomic changes to attractive, attention-getting stimulus events, a faster rate of change of both look duration and of arousal, and more general oscillatory activity in arousal. Finally, 4 sessions of attention training were applied to a subset of the infants (24 trained, 24 active controls), which had the effect of increasing visual sustained attention. No changes in HR responses to stressor were observed as a result of training, but concomitant increases in arousal lability were observed. Our results point to 2 contrasting autonomic profiles: infants with high autonomic reactivity to stressors show short attention durations, whereas infants with lower autonomic reactivity show longer attention durations and greater arousal lability.