The functional role of dorsal medullary inspiratory neurons with excitatory input from lung stretch receptors (R beta neurons) is a matter of controversy. The present study, performed on chloralose-anesthetized and paralyzed cats, ventilated mainly with a phrenic-controlled servorespirator, deals with the spinal projection of these neurons, a property which suggests their involvement in the efferent part of the medullary respiratory complex. Out of 37 inspiratory neurons which could be excited antidromically following microstimulation within the contralateral C6 phrenic nucleus (latency 2.0 ms +/- 0.4, S.D.), 17 were classified by the 'no-inflation' test as R beta. Intracellular recording of synaptic potentials from phrenic motoneurons using the 'spike-triggered averaging' technique were made. In 10 phrenic motoneurons, the averaging revealed individual EPSPs (peak amplitude 150 +/- 110 microV, rise time 0.5 +/- 0.2 ms) time-locked to action potentials of 5 out of 7 R beta neurons tested. Cross-correlation of the R beta neurons firing with the activity of C5 and C6 phrenic rootlets indicated a divergence of excitatory action within the phrenic nucleus. For comparison, in 3 phrenic motoneurons individual EPSPs were recorded when the activity of 3 R alpha cells was used to trigger the averaging. It is concluded that at least some R beta neurons, similarly to R alpha neurons, project to the contralateral phrenic nucleus and can make monosynaptic excitatory connections with phrenic motoneurons. The finding that individual EPSPs were similar when averaging was triggered by the activity of either R beta or R alpha neurons provides new evidence for our earlier hypothesis that bulbospinal inspiratory neurons of the solitary tract nucleus may be subdivided into two categories only on a quantitative basis.