Background Respiratory Sinus Arrhythmia Reactivity (RSA-R) correlates both positively and negatively with externalizing behavior in autistic individuals. These inconsistencies may result from task-based differences. Furthermore, RSA-R is understudied in young autistic children, despite particular challenges with externalizing behavior. This pilot study measured RSA-R in 4-to 6-year-olds, across two time-points, using four validated tasks with matched baseline and challenge periods. Methods RSA and parent-reported externalizing behavior were collected from 17 children (Mage=5.57 years). RSA-R was measured by the difference score of the challenge task minus its corresponding comparison task. Correlations were computed to evaluate relationships between RSA-R and behavior. Results RSA was reliably measured for 3/4 tasks (.694 ≤ ICCs ≤ .896). Only RSA-R during a social task correlated with externalizing behavior. These results support using a battery that measures a range of challenges, differing in social demands, to characterize how arousal contributes to emotion regulation demands among young autistic children.