1. The electrocardiogram was recorded for 3 min during spontaneous respiration in 70 subjects aged 15-86 years who were in sinus rhythm. Using a signal-averaging approach, the presence of respiratory variation of P-P intervals was analysed by multiple regression against a cosine function (cosinor analysis). 2. By cosinor analysis the phase of respiration when the intervals were longest was determined, together with the amplitude of the variation of the intervals around their mean value. 3. Respiratory variation of P-P intervals (respiratory sinus arrhythmia) was demonstrated in 84% of subjects; its amplitude decreased with age and respiratory rate. On average, the duration of P-P intervals varied by 2.8% around the mean, and the maximum duration occurred around the time of end-expiration. 4. By cosinor analysis, and allowing for variation of heart rate, P-R intervals showed an independent respiratory variation in 39% of cases, and its average amplitude was 1.2% around the mean. 5. The respiratory variation of P-R and P-P intervals showed similar phase relationships to respiration, suggesting that during respiration there is parallel alteration of sinoatrial and atrioventricular node function.