1. In 50 subjects with atrial fibrillation we have attempted to demonstrate variation of ventricular rate with respiration, as evidence of cardioregulatory reflex activity. 2. The electrocardiogram was recorded for 3 min during spontaneous respiration. The presence of respiratory variation of R-R intervals was analysed by multiple regression against a cosine function (cosinor analysis), making it possible to determine the phase of respiration when the intervals were longest. 3. Variation in ventricular rate with respect to respiration was demonstrated ( P < 0.05) in seven (14%) cases. On average, R-R intervals were longest at the end of inspiration; this contrasts with sinus rhythm where P-P, P-R and R-R intervals are longest around the time of end-expiration. 4. These results suggest that in atrial fibrillation the beat-to-beat ventricular rate may be under the influence of cardioregulatory reflexes, but the effect of respiration is weak and paradoxical.