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Background: Irritability is a common and impairing occurrence in autistic youth, yet the underlying mechanisms are not well-known. In typically developing populations, differences in frustration response have been suggested as important driver of the behavioural symptoms of irritability. Research exploring the role of frustration response as a risk factor for irritability in autistic populations is limited and often uses parent report or observer ratings; objective measures of frustration response appropriate for use in autistic populations are required to advance the field., Methods: In the current study, fifty-two autistic adolescents aged 13-17 years from a population-based longitudinal study completed an experimental task designed to induce frustration through exposure to periods of unexpected delay. Behavioural (number of button presses) and physiological (heart rate; HR) metrics were collected during delay periods. Irritability was measured using the parent-rated Affective Reactivity Index (ARI). Analyses used mixed-level models to test whether irritability was associated with different slopes of behavioural and physiological response to experimentally induced frustration during the task. Age and baseline HR (for the physiological data only) were included as covariates., Results: Analyses showed a marginal association between irritability and the slope of behavioural response (incident rate ratio (IRR) =.98, p=.06), and a significant association with the slope of physiological response (b=-.10, p=.04); higher levels of irritability were associated with a dampened behavioural and physiological response, as indicated by flatter slopes of change over the course of the task. The pattern of results largely remained in sensitivity analyses, although the association with physiological response became non-significant when adjusting for IQ, autism symptom severity, and medication use (b=-.10, p=.10)., Conclusions: Results suggest that the current experimental task may be a useful objective measure of frustration response for use with autistic populations, and that a non-adaptive response to frustration may be one biological mechanism underpinning irritability in autistic youth. This may represent an important target for future intervention studies., (© 2021. The Author(s).)