1. Sufficient isolated extrasystoles were recorded during a period of quiet breathing in nine patients to test the null hypothesis that there is no within-breath influence of respiration on the triggering of extrasystoles.
2. In five patients a highly significant respiratory modulation of the firing of extrasystoles was seen. The frequency distribution of extrasystoles during the respiratory cycle appeared to be approximately sinusoidal.
3. The phase relations varied with respiratory frequency, and the peak incidence occurred relatively later in the cycle when breaths were shorter: always at about 2 s after the start of inspiration.
4. An autonomic reflex mechanism, equivalent to that causing sinus arrhythmia, appears to be the most likely explanation.