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 mathematically remove the portion of cardiac-related power in MSNA autospectra that was attributable to its linear relationship to the 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 the ECG-MSNA interval (delay from R wave to peak of cardiac-related burst) on the time scale of respiration in four subjects. In these subjects, as well as in some in which the interval appeared to change randomly, there was an inverse relationship between the 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.