The connection between inspiratory neurones in the ventrolateral nucleus tractus solitarius (n.t.s.) and intercostal motoneurones was examined. 2. Descending axonal projections to contralateral T3-T5 spinal segments were found for 110 of 142 (77%) ventrolateral n.t.s. neurones examined. 3. Antidromic mapping was used to locate the axons of thirty-nine ventrolateral n.t.s. neurones in T4, and evidence for axon collaterals was found for thirty-two of forty-seven (68%) neurones examined. Axon collaterals were found in both T3 and T4 for four of nine neurones examined and in T3, T4 and T5 for two of three neurones examined. 4. Cross-correlation histograms were calculated for sixty-five ventrolateral n.t.s. neurones with the contralateral intercostal nerves. Peaks in the cross-correlograms were assessed for significance by calculating k, the ratio of the peak bin count to the mean bin count. Significant peaks (k ratios 1.07-1.24, mean 1.15) were found for twenty-eight (39%) cross-correlograms. Twelve of thirty-three (36%) were for the whole external intercostal nerve, ten of twenty-seven (37%) were for the whole internal intercostal nerve and six of eleven (54%) were for external intercostal nerve filaments. 5. Six of the cross-correlogram peaks were less than or equal to 1.2 ms in width at a level half-way between the peak and the mean bin count. The rest ranged from 2.0 to 4.6 ms (mean 3.0 ms). 6. Intracellular recordings from either internal or external intercostal motoneurones were made and averages of the intracellular potentials were computed using ventrolateral n.t.s. neurone spikes as triggers. 7. Thirty-two spike-triggered averages were computed for pairings between nineteen ventrolateral n.t.s. neurones and thirty-two intercostal motoneurones (twenty-five internal, seven external). Fast-rising, short-lasting depolarizations indicative of a monosynaptic e.p.s.p. were found for five ventrolateral n.t.s. neurones. 8. The characteristics of the cross-correlogram peaks were considered with respect to the e.p.s.p. shapes and it was concluded that the intercostal motoneurones receive a significant monosynaptic excitation from ventrolateral n.t.s. neurones.
