Plasma levels of norepinephrine (NE) vary rhythmically in humans and animals with an ultradian (shorter than 1 day) periodicity. We repeatedly measured plasma NE levels, blood pressure, cardiac output, and total peripheral resistance in nine normal resting subjects over 5 h. Plasma NE correlated with total peripheral resistance (Z = 0.322, P < 0.0002) and inversely with cardiac output (Z = -0.276, P < 0.0002) for the nine subjects overall. The correlations were strongest in subjects with the most spontaneous variability in total peripheral resistance. These findings suggest that spontaneous oscillations in plasma NE levels reflect alterations in sympathetic nervous activity to resistance blood vessels. The negative correlation between cardiac output and plasma NE levels may result from the very minor cardiac NE spillover into plasma and the inverse relationship between cardiac output and total peripheral resistance.