Indices of physiological regulation (i.e., resting respiratory sinus arrhythmia [RSA] and RSA suppression) and observed fearfulness were tested as predictors of empathy-related reactions to an unfamiliar person's simulated distress within and across 18 (T1, N = 247) and 30 (T2, N = 216) months of age. Controlling for T1 helping, high RSA suppression and low fearfulness at T1 predicted T2 helping. In a structural model, empathic concern was marginally positively related to resting RSA at both assessments whereas personal distress was related to RSA suppression within time (marginally positively at T1 and significantly negatively at T2). Fearfulness was associated with self-oriented, distress-related reactions within time. Comfort seeking (an index of personal distress) declined in mean level with age whereas helping increased, and both behaviors exhibited differential continuity (as did resting RSA). Individual, as well as developmental, differences in the types of reactions that young children exhibit when witnessing others' suffering and distress were discussed. (Contains 4 tables, 1 figure and 2 notes.)