In anesthetized cats, with vagi cut and the spinal cord severed at the C8 level, phrenic motor and/or sensory discharge was recorded. Small afferent phrenic fibers were identified through their activation by lactic acid, hyperosmotic NaCl solution, or phenyl diguanide. They exhibited a spontaneous but irregular low-frequency discharge. Block of their conduction by procaine had no effect on eupneic motor phrenic activity. Large afferent phrenic fibers showed a spontaneous rhythmic discharge, and cold block (6 degrees C) of these fibers significantly prolonged the phrenic discharge time (Tphr) and total breath duration (TT) during eupnea. The stimulation of all afferent phrenic fibers lowered the impulse frequency of phrenic motoneurons (f impulses) and shortened both Tphr and TT. When the stimulation was performed during cold block all of the effects on phrenic output persisted, but changes in timing were less pronounced. Under procaine block, only the effects of phrenic nerve stimulation on Tphr persisted. These results suggest that both large and small afferent phrenic fibers control the inspiratory activity with a prominent role of small fibers on phrenic motoneuron impulse frequency.