1. The contractile activity of muscularis mucosae strips isolated from cat lower esophageal sphincter (LES) was studied. 2. LES muscularis mucosae strips were characterized by spontaneous phasic contractions. 3. Acetylcholine (ACh) at a threshold concentration of 10(-8) M dose-dependently increased the basal tone and decreased the amplitude of the phasic contractions. Noradrenaline (NA) also increased the basal tone but at a higher threshold concentration (10(-6) M). 4. Nicotine (N) at concentrations of 5 x 10(-5)-10(-4) M induced a relaxation of the LES muscularis mucosae. The N-induced relaxation was tetrodotoxin (TTX) (10(-6) M) sensitive and was insignificantly reduced after guanethidine (5 x 10(-6) M) plus scopolamine (5 x 10(-6) M) or propranolol (10(-6) M). The N-induced relaxation was strongly decreased by the blocker of nitric oxide (NO) synthesis N omega Nitro-L-Arginine (L-NNA) (10(-4) M). The relaxation was restored after addition of L-arginine (10(-3) M). 5. These results suggest the involvement of NO in the N-induced relaxation of the muscularis mucosae.