In brain stem-spinal cord preparations isolated from newborn rats, we examined synaptic connections to inspiratory neurons from preinspiratory (Pre-I) neurons, by spike-triggered averaging. Potentials were simultaneously recorded intracellularly from inspiratory neurons, and extracellularly from Pre-I neurons in the ipsilateral ventrolateral medulla. The Pre-I neuron pulses were used to trigger computer averaging of potential changes. A total of 16 pairs were analyzed, and EPSP-like depolarizing potentials locked to the triggering Pre-I spikes were obtained from three pairs. Latency from the Pre-I spikes was 3.4-4.5 ms, and the amplitude was 0.07-0.4 mV. These connections were concluded to be monosynaptic, because the antidromic latency was 3.6 +/- 1.5 ms. The present results suggest the presence of excitatory synaptic connections from Pre-I neurons to inspiratory neurons in the ventrolateral medulla. These excitatory synaptic connections are probably important in the initiation of inspiratory burst activity.