Spinal cords were maintained in vitro and suction electrodes used to record activity in lumbar 4 or 5 ventral roots. Stimulation of the latero-ventral aspect of the thoracic cord elicited fast and slow responses on the same and on the opposite side of the cord. There were 5 distinct responses: ipsilaterally a short latency (d ISL), a polysynaptic and a slow response, and contralaterally a fast (d CON FAST) and a slow response. The largest amplitude component, d ISL, may arise from stimulation of propriospinal neurones; the other responses may arise from stimulation of descending pathways. The slow responses had half decay times of 13-15 s and required a high intensity stimulus to elicit a maximal response. All 5 responses were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione suggesting that kainate/AMPA receptors were involved in their generation. In addition, NMDA receptors were involved in generation of the slow responses. Potentiation of certain responses by the 5-HT2 antagonists, ketanserin, ritanserin and Lilly 53837, indicated that endogenous 5-HT was exerting a modulatory depression of these responses. In addition to eliciting the 5 responses, thoracic cord stimulation caused an inhibition of segmental reflexes evoked from the lumbar dorsal root. Exogenous 5-HT, 8-hydroxy-2-(di-n-propylamino) tetralin, 5-carboxamidotryptamine, dipropyl-5-carboxamido-tryptamine and methysergide depressed all or some of the descending responses. Blockade of adrenoceptors using yohimbine, idazoxan, prazosin or propranolol had no unequivocal effect suggesting that the release of endogenous catecholamines was minimal. Clonidine was a potent depressant of the slow responses.(ABSTRACT TRUNCATED AT 250 WORDS)