The reflex control of pulse interval during erect bicycle exercise was studied in nine normal subjects aged 18 to 28, breathing an oxygen-rich mixture. Rises in the directly measured arterial pressure were produced by intravenous injections of phenylephrine, and systolic pressure was correlated with the following pulse interval. The slope of the systolic pressure-pulse interval relation was used to express reflex sensitivity. With increasing exercise, the reflex sensitivity decreased progressively so that at a level of exercise which produced a heart rate of 150/min there was no reflex cardiac slowing in response to a provoked rise of pressure. The reduction in reflex sensitivity was studied during the changes from rest to exercise and vice versa. Reflex sensitivity decreased within 5 seconds of the onset of exercise.