An inverted U-shaped pattern in heart rate (HR) and a U-shaped pattern in heart rate variability (HRV) are easily recognizable when individuals experience any acute stressor. How cardiac complexity (scaling and entropy) changes under acute stress is not well known. Psychologically, emotion regulation (ER) style is likely to influence the individual\'s specific behavioral response when affronting stress. This study tested whether adolescents with distinct ER styles would show different patterns of linear and nonlinear cardiac changes under stressful conditions. We predicted less autonomic flexibility for adolescents with a highly negative emotional regulation (HNER) style (n = 10) than for those adolescents with a highly positive emotional regulation (HPER) style (n=10). Further, associations between linear and nonlinear measures during each condition were examined for each group. Repeated measures ANOVAs revealed that HR and HRV changed according to the predicted pattern. Higuchi\'s fractal dimension and Sample Entropy followed a U-shaped pattern, whereas the short-term scaling exponent followed the reverse pattern. Cardiac changes across conditions were larger in the HPER group. Significant associations between linear and nonlinear measures were found in the HPER group but not in the HNER group. Results are cautiously discussed within a multiscale framework of fluctuations of the different cardiac features.