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The purpose of the present study was to examine differences in infants' negative reactivity and use of regulation strategies during states of wakefulness and fatigue. Despite progress in building a theoretical foundation for the development of emotion regulation, studies designed to support current theories have only recently emerged in the literature. Moreover, researchers have not addressed how an infant's ability to manage negative reactivity changes based on current state of arousal. Five mildly stressful episodes were included in the testing session: colorful toys were taken from infants and sealed in a plastic jar, a remote-controlled toy approached infants, mothers left the room for a brief period, mothers prohibited infants from touching an attractive toy, and infants played while mothers completed questionnaires. A variety of infants' responses were recorded, including average negative reactivity, latency to first negative response, peak negative intensity, looks to mother, looks to experimenter, self-comforting, proximity-seeking, toy exploration, and direct attempts to eliminate the stressor. The following maternal behaviors were also recorded: facial expressions, vocal tone, physical intervention with infants, and individual verbalizations. Mothers were asked to complete two temperament questionnaires prior to the session, and they responded to a number of pre- and post-session questions about their infants during the session. Mothers were given a certificate and a toy for their infants. Several of the findings from this study supported hypotheses that infants who were tired did, in fact, exhibit different levels of negative reactivity and different types of regulation strategies during the testing session than infants were alert. For example, infants in the fatigued condition displayed higher average negative reactivity during the mother separation episode and the attractive toy episode. Significant group differences in emotion regulation were found only for proximity-seeking behaviors in the mother separation episode, in which fatigued infants were more likely to seek proximity to mothers than alert infants. Means for reactivity and regulatory behaviors, although not often statistically significant, were consistently in the expected direction. Future studies, conducted in both the home and the lab, should continue to address the effects of fatigue on infants' ability to regulate emotions.