Respiratory Sinus Arrhythmia Reactivity (RSA-R) correlates both positively and negatively with externalizing behaviour in autistic individuals. These inconsistencies may result from task-based differences. This pilot study measured RSA-R in 4-to 6-year-olds, across two timepoints, using four validated tasks with matched baseline and challenge periods. Social, cognitive, sensory and emotional tasks were employed to evaluate the use of a domain-specific approach in measuring RSA-R in young autistic children. RSA and parent-reported externalizing behaviour were collected from 16 children (M[subscript age] = 5.60 years; 13 male; 12 White/Caucasian; 15 non-Hispanic/Latine). RSA-R was measured by the difference score of the challenge task minus its corresponding comparison task. Correlations were computed to evaluate associations between RSA-R and behaviour. RSA was reliably measured for 3/4 tasks (0.694 [less than or equal to] intraclass correlation coefficients [ICCs] [less than or equal to] 0.896). Only RSA-R during a social task correlated with externalizing behaviour. 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.