Loss of autonomic balance characterized by increased sympathetic activity and decreased vagal activity has been implicated as a major cardiovascular risk factor. Aspirin's cardioprotective abilities involve a multitude of physiologic processes. However, the effects of aspirin on cardiac autonomic activity are unknown. In a double-blind crossover study, 22 subjects randomly received either aspirin or placebo in the amounts of 325 mg with each meal (three times per day) over a 2.5-day period. The total amount of aspirin ingested was 2,275 mg, which resulted in plasma levels of 3.3 mg/dl. At the conclusion of each treatment, subjects were evaluated for autonomic physiology activity using standard autonomic tests. Power spectral analyses of the electrocardiograms were used to delineate autonomic function. A 2 x 4 repeated measures analysis of variance revealed significant and favorable changes in autonomic activity after the use of aspirin. Specifically, at rest high-frequency (HF) power was significantly higher (mean, 1,090 + 1,463.5 msec2) compared with the placebo (mean, 692 742 msec2) (p <0.05). Low-frequency (LF) power was significantly reduced (mean, 963 745 msec2) after aspirin compared with placebo (mean, 1,100 906 msec2). After the aspirin treatment, a significantly lower LF-to-HF power ratio (mean, 1.7 2 msec2) was noted at rest when compared with the placebo (mean, 2.5 2.7 msec2) (p <0.05). Similar significant trends were seen during the sustained isometric contraction after aspirin therapy for HF power (mean 210 2.15 msec2) compared with placebo (mean, 213 184 msec2) (p <0.05). Accordingly, the LF-to-HF power ratio was lower as well when compared to placebo treatment (mean, 2.3 3.5 msec2) (mean, 5.3 8.4 msec2) (p <0.05). No differences were found in breathing rates for hemodynamic variables between any of the protocols. The significant reduction of LF-to-HF ratio, a marker of sympathovagal balance, for both protocols appeared to be largely due to a withdrawal of LF modulation and concomitant but lesser increase in HF modulation. Favorable alterations in autonomic outflow through prostaglandin inhibition may be one of the mechanisms by which low therapeutic amounts of aspirin provide prophylactic cardioprotection.