Studies were performed on anesthetized, paralyzed, artificially ventilated cats. Phrenic (C5) efferent activity, dorsal respiratory group (DRG) neuron activity in the vicinity of the medullary solitary tract complex, and thoracic dorsal root compound action potentials were recorded during electrical stimulation of intercostal and lumbar nerves. DRG neurons were identified by their firing pattern and response to lung inflation. Phrenic activity (PA) was inhibited by stimulating external intercostal nerves T3-T10, internal intercostal nerves T3-T12, lateral branch of the main intercostal nerves T6-T12, or lumbar nerves 1-2. Stimulation of lower (T9-T11) intercostal or lumbar nerves produced a short duration (10-20 msec) facilitation of PA prior to the inhibition. Facilitation and inhibition of PA were correlated with recruitment of afferent fibers from muscle proprioceptors. Inspiratory neurons (I alpha and I beta) in the DRG were inhibited simultaneously with PA regardless of the nerves stimulated. DRG neurons which fired in phase with lung inflation (P cells) were unaffected by nerve stimulation even though PA was inhibited. Lower intercostal nerve (T9-T11) stimulation produced a brief facilitation of medullary neuron activity simultaneous with facilitation of PA. It is concluded that intercostal and abdominal muscle proprioceptor afferents, and perhaps cutaneous afferents, reflexly alter the activity of DRG inspiratory neurons (I alpha, I beta) which drive the phrenic motoneurons. The inhibitory effect is not via P cells but may be via other interneurons in close proximity to the I cells.