The autonomic flexibility model has proven to be a useful theoretical tool relating reductions in physiological variability found to accompany anxiety and concomitant reductions in behavioral (e.g., cognitive and emotional) flexibility. The present study aimed to extend the autonomic flexibility model through the inclusion of individual differences in the sensitivity of the independent motivational systems presumed to underlie anxiety and impulsivity, namely the behavioral inhibition and activation systems (BIS/BAS; Gray, 1994). Contrary to the predicted inverse relationship between BIS sensitivity and measures of physiological variability, findings suggest BAS sensitivity is associated with increased trait-like vagally mediated heart rate variability across diverse tasks as well as greater flexibility in responding within tasks. Numerous BIS*BAS interactions emerged as significant predictors of trait reactivity. Results are discussed in terms of the interface between (1) mesolimbic dopaminergic projections to the nucleus accumbens and (2) the network of central nervous system structures believed to play a large role in controlling peripheral physiology. / Ph. D.