TO elucidate synaptic interactions between expiratory (Exp) and other respiratory neurons in the ventrolateral medulla of brain stem–spinal cord preparations from newborn rats, we analyzed postsynaptic potentials in Exp and a subclass of inspiratory (Insp) neurons using whole-cell recordings. About 72% of the Exp neurons (Exp-p-i) showed Clγ-dependent synaptic inhibition during the Insp and pre/post-Insp phases, corresponding to the active phase of the Insp and pre-inspiratory (Pre-I) neurons, respectively. The other 28% neurons (Exp-i) received Cl−-dependent inhibition during the Insp phase only. Some Insp neurons showed reversed IPSPs during the active phase of Exp-p-i after Cl− loading. We suggest the existence of inhibitory connections from Pre-I and Insp to Exp and from Exp to Insp neurons. Basic synaptic connections among respiratory neurons similar to those in intact adult mammals may already exist in newborn rats.