We studied the effects on sinus rate and atrioventricular (AV) conduction of electrical stimulation from a 12-polar electrode catheter advanced into the right pulmonary artery of 21 anesthetized dogs. In each experiment, the distal tip of the electrode catheter was positioned at a standard fluoroscopic site, and a sequence of bipolar electrograms was recorded during sinus rhythm from the 11 adjacent catheter electrode pairs using a standardized technique. Within each sequence of electrograms, a characteristic change in the polarity of the atrial complexes was identified at a site in the proximal right pulmonary artery. This recording site was labeled the site of initial polarity transition. Stimulus-strength response testing was performed from each catheter electrode pair during spontaneous sinus rhythm and during atrial fibrillation sustained by rapid atrial pacing. The least stimulus strengths required to slow sinus rate or to depress AV conduction were obtained using an electrode pair at a proximal right pulmonary artery site identified as the optimal stimulation site. This stimulation site was at, or immediately proximal to, the recording site of initial polarity transition. Stimulation distal to the site of initial polarity transition precipitated atrial fibrillation using stimulus strengths which were very low compared to stimulus strengths required to precipitate atrial fibrillation at more proximal sites. Negative chronotropic and negative dromotropic effects persisted throughout 5-minute periods of stimulation from the optimal stimulation site and could be modulated by varying stimulus parameters. Using neurophysiological and neuropharmacological techniques, we demonstrated that these effects were produced by stimulation of preganglionic parasympathetic efferent nerve fibers. Circ Res 46: 48-57, 1980