Two kinds of electrical activity can be recorded in the pyloric antrum with extracellular electrodes: an initial triphasic spike-like potential and sometimes a second potential. Contractions of the antrum were accompanied by the second (negative) potential wave. The initial potential was propagated. An initial potential preceded each second potential during contractile activity and appeared to initiate it. These patterns of electrical activity were compared with those in the duodenum previously reported. Cholinergic stimulants (acetylcholine, methacholine, carbamylcholine, and nicotine) infused intra-arterially in threshold amounts (0.1 to 1 μg) produced in the antrum a premature initial wave, one or more second potentials with contractions, or both effects. The nature of the response depended upon the timing of the infusion in relation to the initial wave and the dose of drug used. Premature initial waves were capable of antiperistaltic conduction. Doses of acetylcholine higher than threshold caused both premature initial waves and a series of second potentials. Excessive doses (>1 μg) caused disruption of initial waves. Both muscarinic and nicotinic cholinergic receptors were identified.Catecholamines infused intra-arterially in threshold amounts temporarily obliterated second potentials and associated contractions in the antrum. Pharmacological analysis suggests that alpha receptors were involved. Beta inhibitory receptors were also present. Higher concentrations of catecholamines, or atropine, may produce a series of rapid initial waves without second potentials or contractions, which have been designated the "sympathetic dominance pattern". Production of this pattern by catecholamines probably resulted from actions on alpha receptors. Noradrenaline and adrenaline suppress fast spike potentials and contractions in the duodenal bulb. In larger amounts they decrease slow-amplitude potentials and increase their frequency. Isopropylnoradrenaline in most experiments caused contractions and fast spike potentials in the duodenal bulb when given in small or moderate amounts. The inhibitory effects of catecholamines were on alpha receptors, but beta receptors with excitatory effects may also be present.