This study investigated relationships between laboratory task-related heart rate reactivity and ambulatory heart rate responses and variability during daily life. Sixty-two healthy, young men completed one laboratory session consisting of pretask rest, reaction time, video game, and mental arithmetic, and one 24-hour ambulatory heart rate monitoring day. After controlling for ambulatory physical activity, laboratory reactivity measured as basal-to-task increase correlated significantly with ambulatory minimum, mean, and maximum increases from basal during wakefulness and with awake mean square successive difference. After controlling for ambulatory physical activity, laboratory reactivity measured as pretask-to-task increase correlated significantly only with awake mean square successive difference. These results support the hypothesis that individual differences in heart rate responses to laboratory tasks reflect similar differences in heart rate responses and variability during daily life. Results are compared in their degree of support for two theoretical models describing the generalization of individual differences in laboratory reactivity to reactivity during daily life.