This study quantified the effect of interrupting the descending input to the sympathetic preganglionic neurons upon the dynamic behavior of arterial blood pressure (BP) in the unanesthetized rat. BP was recorded for ~4 hour intervals in six rats in the neurally intact state and in the same animals following complete spinal cord transection (SCT) between T₄ and T₅. In the intact state power within the frequency range of 0.35 - 0.45 Hz was 1.53 ± 0.38 mm Hg²/Hz (mean ± SD by Fast Fourier Transform). One week after SCT power within this range decreased significantly (p < 0.05) to 0.43 ± 0.62 mm Hg²/Hz. To test for self-similarity before and after SCT data were analyzed using a wavelet (i.e., functionally a digital bandpass filter) tuned to be maximally sensitive to fluctuations with periods of approximately 2, 4, 8, 16, 32 or 64 seconds. In the control state all fluctuations with periods 4 seconds conformed to a "self-similar" (i.e., fractal) distribution. In marked contrast, the oscillations with a period of ~2 sec. (i.e., ~0.4 Hz) were significantly set apart from those at lower frequencies. One day and seven days after the complete SCT, however, the BP fluctuations at ~0.4 Hz now also conformed to the same self-similar behavior characteristic of the lower frequencies. We conclude that (a) an intact sympathetic nervous system endows that portion of the power spectrum centered around ~~0.4Hz with properties (e.g., a periodicity) that differ significantly from the self-similar behavior that characterizes the lower frequencies and (b) even within the relatively high frequency range at 0.4 Hz self-similarity is the "default" condition after eliminating sympathetic influences.