The effect of isosorbide dinitrate (5 mg, sublingually) and of atropine (12 micrograms/kg i.v.) on the lower esophageal sphincter pressure of chagasic patients with esophageal involvement and of control subjects was studied by the manometric method. The pressure was measured at 5 minute intervals for 60 minutes after drug administration. When the effect of isosorbide dinitrate was studied, basal sphincter pressure (mean +/- SEM) was 15.3 +/- 1.9 mmHg in chagasic patients (N = 15) and 25.4 +/- 5.6 mmHg in controls (N = 9) (P greater than 0.05). Isosorbide dinitrate reduced sphincter pressure between 10 and 20 minutes and at 40 minutes in controls, and froM 5 to 60 minutes in chagasics (P less than 0.05). The reduction was more intense in chagasics throughout the time of measurement (P less than 0.05). When the effect of atropine was studied, basal sphincter pressure was 20.6 +/- 1.8 mmHg in chagasic patients (N = 14) and 23.7 +/- 2.5 mmHg in controls (N = 9) (P greater than 0.05). Atropine reduced sphincter pressure both in chagasics and controls, but more intensely so in controls during the first 30 minutes of the study (P less than 0.05). The action of the musculature itself may be the main factor in the maintenance of sphincter pressure in chagasics, with secondary participation of the cholinergic excitatory system.