We investigated the effects of treatment with the tricyclic antidepressant (TCA) imipramine in eight children in supine and standing postures. We used 256 seconds of real-time data for the analyses. Spectral analysis showed a significant decrease of high-frequency (HF) power (0.15-0.5 Hz), especially in the standing posture. Low frequency (LF) HF ratios were significantly higher in the standing posture after treatment, suggesting increased sympathovagal interaction. We also obtained the nonlinear measures of fractal dimension (FD), and approximate entropy (APEN). Although the FD of heart rate was significantly lower in the standing posture, APEN was significantly decreased after treatment in either posture and was the most sensitive measure in this study. These findings suggest a decreased cardiac vagal function with a relative increase in sympathetic responsiveness, which may in part be related to the cardiotoxicity of these drugs. These findings are discussed in relation to the cardiovascular side effects of TCAs.