Right ventricular bypass operations were performed in dogs by right atrium--pulmonary artery (RA-PA) conduits with closure of the tricuspid valve. Atrial fibrillation (AF) was induced in prebypass (control) and postbypass (postconduit) studies. Six dogs tolerated the procedure and had postoperative sinus rhythm (SR). In the postconduit studies, AF resulted in significant decline in both mean arterial pressure (73 +/- 2 to 65 +/- 2 mm Hg [mean +/- SEM], p less than 0.05) and pulmonary blood flow (1,050 +/- 160 to 880 +/- 110 ml/min, p less than 0.05). However, there were no significant differences between the magnitude of these changes and those during control studies. Mean right atrial pressure was markedly elevated (17.3 +/- 1.3 mm Hg) during SR after the conduit operations, but it did not increase significantly with AF. In volume load studies, pulmonary blood flow increased in proportion to rises between 15 and 30 mm Hg in mean right atrial pressure. The results suggest that right atrial contraction is not critical to the maintenance of pulmonary blood flow following RA-PA conduit operations.