Background: We previously showed that lysozyme (Lzm-S) derived from leukocytes caused myocardial depression in canine sepsis by binding to the endocardial endothelium to release nitric oxide (NO). NO then diffuses to adjacent myocytes to activate the guanosine 3', 5' monophosphate (cGMP) pathway. In a canine right ventricular trabecular preparation (RVTP), Lzm-S also decreased the inotropic response to field stimulation (FSR) in which the sympathetic and parasympathetic nerves were stimulated. In the present study, we determined whether the pathway by which Lzm-S caused a decrease in FSR was different from that which resulted in a decline in steady state contraction. Furthermore, we determined whether the decrease in FSR was due to a decrease in sympathetic stimulation or enhanced parasympathetic signaling. Methods and Results: In the RVTP, we found that Lzm-S's inhibitory effect on FSR was prevented by nitric oxide synthase (NOS) inhibitors. A cGMP inhibitor also blocked Lzm-S's depressant activity. However, in contrast to the decline in steady state contraction caused by Lzm-S, chemical removal of the endocardial endothelium by Triton X to eliminate endothelial NO release did not prevent the decrease in FSR. An inhibitory G protein was involved in Lzm-S's effect, since FSR could be restored by treatment with pertussis toxin. Atropine prevented Lzm-S's decline in FSR, while _1-2 adrenoceptor function was not impaired by Lzm-S. Conclusion: These results indicate that the decrease in FSR caused by Lzm-S results from a non-endothelial release of NO. NO then acts through inhibitory G protein to enhance parasympathetic signaling.