Effects of tetraethylammonium and 4-aminopyridine on the canine sinus node pacemaker cells and on canine atrial muscle cells were investigated in this study. Atria isolated from young dogs were each perfused through the sinus node artery with physiologic buffer solution. Action potentials were recorded with microelectrodes and contractions were monitored with a displacement transducer. High concentrations of tetraethylammonium (10 mmol/l) or 4-aminopyridine (1.0 mmol/l) produced atrial arrest within 15 min. However, lower concentrations (1.0 mmol/l for tetraethylammonium; 0.01 mmol/l for 4-aminopyridine) perfused for only 10 s produced brief (1 ± 0.5 min) sinus tachycardia and stronger contractions. When atropine was present, the level of sinus tachycardia tripled; when propranolol was present, both tetraethylammonium and 4-aminopyridine had only a slowing effect. The contractile responses were similarly affected. When both atropine and propranolol were present, tetraethylammonium increased sinus rate and contraction amplitude and 4-aminopyridine increased sinus rate. Action potentials of atrial cells exposed to tetraethylammonium or 4-aminopyridine had less negative diastolic transmembrane potentials, slower upstrokes, and longer durations. All these changes could be reversed by acetylcholine. With low (0.42 mmol/l) extracellular [K+], tetraethylammonium and 4-aminopyridine had no effect on sinus rate but did increase contraction amplitude by 50%. We conclude that after accounting for the effects of catecholamines and acetylcholine released in canine atrial muscle, tetraethylammonium and 4-aminopyridine directly increase sinus rate and tetraethylammonium directly increases contraction amplitude. © 1981.