Factors regulating the putative peptide neurotransmitter, substance P, were examined in the adult rat sympathetic superior cervical ganglion (SCG). Surgical section of the preganglionic nerves resulted in a 55% increase in ganglion content of SP-like immunoreactivity (subsequently termed SP). Treatment with chlorisondamine, which blocks nicotinic ganglionic transmission, also increased SP, reproducing the effects of decentralization. Conversely, treatment with phenoxybenzamine, which reflexly increases sympathetic activity, reduced ganglion SP. Axotomy of the postganglionic nerves did not alter ganglion SP. Finally, treatment of rats as neonates with 6-hydroxydopamine, which selectively destroys principal ganglion neurons, profoundly decreased SP in the adult SCG. Our observations suggest that impulse activity of preganglionic nerves decreases ganglion SP in principal ganglion neurons through a transsynaptic process. Viewed in conjunction with previous work, our studies suggest that transsynaptic stimulation has opposite effects on SP and noradrenergic transmitter characters in the SCG.