To determine the acute effects of atrial fibrillation on regional blood flow, measurements were made in awake dogs with this arrhythmia induced and sustained by rapid atrial stimulation. Atrial fibrillation reduced cardiac output (from 3.7 +/- 0.3 to 3.0 +/- 0.2 L/min, P less than 0.05), but mean aortic and left atrial pressures were not changed. Although average ventricular myocardial blood flow remained the same, dogs with an average basal myocardial blood flow less than 106 mL/min/100g showed an immediate increase (from 85 +/- 5 to 120 +/- 9 mL/min/100g, P less than 0.05), whereas those with a higher basal value showed a decrease (from 144 +/- 14 to 110 +/- 18 mL/min/100g, P less than 0.05). Moreover, change in myocardial blood flow with atrial pacing, at a rate equal to the average ventricular rate of atrial fibrillation, was directionally similar to that found during atrial fibrillation. However, left atrial myocardial blood flow increased significantly both during atrial fibrillation and pacing. Sustained atrial fibrillation resulted in a fall in splanchnic and renal cortical flow. Brain blood flow also decreased during atrial fibrillation. While the fall in cerebral blood flow was immediately evident, in the cerebellum and brain stem, this decrease was not statistically significant until the 15 min measurement. Also, presence of a ligated common carotid artery did not influence cerebral regional blood flow either under basal conditions or with atrial fibrillation. Thus, in awake dogs the fall in cardiac output that occurs with atrial fibrillation may be accompanied by diverse effects on regional circulations.