Decreased heart rate (HR) variability has been shown to be an independent predictor of poor outcome after acute myocardial infarction. Presumably, both reduced parasympathetic activity and increased sympathetic activity contribute to this observed HR variability response. To elucidate the physiologic contributions of the sympathetic and parasympathetic nervous systems to HR variability, the effect of passive head-up tilt (+70 degrees) was investigated on 4 indexes of HR variability in 17 healthy subjects. The standard deviation of the mean of 512 consecutive RR intervals, a traditional marker of HR variability and a purported index of cardiac parasympathetic neural tone, was compared with the mean difference of 512 consecutive RR intervals, with the maximal expiratory RR interval to minimal inspiratory RR interval ratio (respiratory RR ratio) and with the low- and high-frequency components of the power spectrum of 512 consecutive RR intervals. Passive tilt produced a nonsignificant decrease in the standard deviation of RR intervals. There was, however, a highly significant decrease in the mean difference of consecutive RR intervals and the high-frequency component of the RR-interval spectrum. Both the respiratory RR ratio and the low-frequency component of the RR-interval spectrum increased with tilt. It was concluded that the mean difference of consecutive RR intervals and the high-frequency component of the RR-interval spectrum are potentially superior indexes of "pure" parasympathetic neural tone and may be preferable for future use in cardiovascular studies of autonomic dysfunction.