: Pedal ganglia excised and maintained in culture for up to 2 h, release NO at low levels. The range can vary between 0 to 1.1 nM. Non-stimulated immunocytes do not significantly stimulate ganglionic NO release when incubated with pedal ganglia. However, ganglia exposed to immunocytes that had been previously activated by a 30 min incubation with interleukin 1 beta, release NO significantly above basal levels. In these experiments, 91 +/- 2.5% of the non-stimulated immunocytes exhibited form factors in the 0.72 to 0.89 range (sampled prior to ganglionic addition), whereas 62 +/- 10.3% of the interleukin 1 beta stimulated immunocytes had form factors in the 0.39 to 0.49 range, demonstrating activation. Addition of the nitric oxide synthase inhibitor, L-NAME (10(-4) M), inhibited basal ganglionic NO release as well as that initiated by exposing the ganglia to activated immunocytes. Interestingly, non activated immunocytes, following ganglionic exposure, exhibited activity levels in the 13% range, representing a non significant increase. Cells exposed to interleukin 1 beta had a 65% activity level at the beginning of the experiment, followed by a drop of activity to 19 +/- 3.2% after ganglionic exposure. Repeating this last observation in the presence of L-NAME (10(-4) M), brought the activity level of the immunocytes back to the pre-ganglionic exposure level of activity, demonstrating that ganglionic NO was involved in down regulating immunocyte activity.