We used bispectral analysis to characterize the nonlinear interactions of the respiratory-related (RR), cardiac-related (CR), or 10-Hz rhythms in sympathetic nerve discharge (SND) of urethan-anesthetized cats. Bispectral analysis investigates relationships among frequency triples in the same signal (inferior cardiac postganglionic SND) where the third frequency is the sum of the other two due to quadratic nonlinear coupling. Coupling of the RR and CR rhythms leading to the generation of new components (i.e., modulated frequencies) in SND occurred in 84% of the total cases, whereas the incidence was 71% for the RR and 10-Hz rhythms. The occurrence of such nonlinear interactions implies that the RR, CR, and 10-Hz rhythms are carried to common targets by the same postganglionic sympathetic neurons. Furthermore, we suggest that nonlinear interactions leading to the generation of new frequencies in SND may affect end-organ function beyond the level expected in simple cases of linear superposition of the primary rhythms. This suggestion is supported by our observation that strong coupling of the RR and CR rhythms resulted in appreciable power at the modulated frequencies.