SUMMARY1. The action of muscle contraction on the sensitivity of the cardiac vagal component of the baroreceptor reflex was examined in decerebrate cats.2. The sensitivity of the baroreceptor reflex was expressed as the difference between the maximum prolongation of the R-R interval in response to carotid sinus baroreceptor stimulation and the mean of ten R-R intervals immediately before carotid sinus pressure elevation.3. Muscle contraction elicited by electrical stimulation of L7 ventral roots (50 Hz) significantly reduced the sensitivity of the baroreceptor reflex by reducing the prolongation of the R-R interval from 269 + 31 to 159 + 22 ms.4. Inhibition of the cardiac vagal component of the baroreceptor reflex was seen just 1 s after the onset of contraction and with stimulation frequencies as low as 10 Hz.5. These results show for the first time that changes in the sensitivity of the baroreceptor reflex during exercise result in part from afferent information originating in the contracting muscles.