Individual differences in high-frequency heart rate variability (HRV) have been conceptualized in terms of a greater capacity to self-regulate problematic outcomes, but have also been conceptualized in terms of greater moment-to-moment flexibility. From a self-regulation perspective, higher HRV should be inversely correlated with trait neuroticism and problematic daily outcomes. From a flexibility perspective, high HRV should result in more state-like functioning—that is, functioning that is more contextual and less trait-like in nature. In the latter case, HRV and trait neuroticism may interact to predict problematic outcomes such that neuroticism should be a less consequential predictor at higher levels of HRV. The flexibility perspective was systematically supported in a daily experience-sampling protocol. Implications focus on theories of neuroticism and HRV.(PsycInfo Database Record (c) 2023 APA, all rights reserved)