Using isolated perfused porcine ileum we studied the release of substance P (SP) and neurokinin A (NKA) in response to electrical stimulation of the mixed periarterial nerves and to infusion of different neuroactive agents. Nerve stimulation (8 Hz) had no significant effect on the release of SP and NKA. Nerve stimulation also had no effect on the release of SP and NKA during infusion of atropine (10(-6) M) or phentolamine (10(-5) M), whereas a significant increase (from 8.2 +/- 1.9 to 20.1 +/- 4.6 pmol/l for SP and from 12.3 +/- 2.7 to 34.2 +/- 7.7 pmol/l for NKA, n = 7) was observed during nerve stimulation after pretreatment with both atropine and phentolamine. This increase was abolished by hexamethonium (3 x 10(-5) M). Also acetylcholine infusion causes a significant release of SP and NKA after infusion of both atropine and phentolamine (to 172 +/- 56% and 232 +/- 69% of basal release, n = 7), an effect that was abolished by hexamethonium infusion. Infusion of atropine alone increased the release of SP and NKA significantly (to 337 +/- 92% and 386 +/- 124% of basal output, n = 5). Norepinephrine (10(-6) M) inhibited the release of SP and NKA (to 69 +/- 6% and 80 +/- 6% of basal release, n = 7). Our results suggest that the SP- and NKA-producing neurons receive intrinsic tonic muscarinic inhibitory impulses, extrinsic nicotinic excitatory impulses, and extrinsic adrenergic inhibitory impulses.