We tested the hypothesis that the cardiac-related rhythm in muscle sympathetic nerve activity (MSNA) of humans reflects entrainment of a central oscillator by pulse-synchronous baroreceptor nerve activity. Partial autospectral analysis was used to remove mathematically the portion of cardiac-related power in MSNA autospectra that was attributable to its linear relationship to the electrocardiogram (ECG). In 54 of 98 cases, 15% of cardiac-related power remained after partialization with the ECG; peak residual cardiac-related power was often at a frequency different than heart rate. When assessed on a cardiac-related burst-by-burst basis, there was a progressive and cyclic change in ECG-MSNA interval (delay from R-wave to peak of cardiac-related burst) on the time scale of respiration in 4 subjects. In these subjects, as well as in some in which the interval appeared to change randomly, there was an inverse relationship between ECG-MSNA interval and cardiac-related burst amplitude. However, in 45% of the cases, these parameters were not related. These results support the view that the cardiac-related rhythm in MSNA reflects forcing of a nonlinear oscillator rather than periodic inhibition of unstructured, random activity.