Trigeminal (V) tractotomy and cold block of synaptic transmission in V nucleus caudalis were used to show that caudalis modulates the responses to innocuous and noxious stimuli of single V main sensory-oralis neurones recorded in anaesthetized or decerebrate cats. Cold block caused a reversible depression of mechanosensitivity of 91 of 105 V-thalamic relay and non-relay cells tested; V tractotomy also decreased sensitivity. The possibliity that the effects observed with cold block of caudalis were caused by direct spread of cooling to the main sensory-oralis region, and not by depression of a tonic, net facilitatory influence of caudalis on main sensory-oralis cells, was ruled out by several controls. With cold block of caudalis there also occurred a reversible shrinkage in mechanoreceptive field size and reversible reduction in sensitivity of rapidly adapting and slowly adapting mechanoreceptive neurones. Occasionally no change or an increase in sensitivity occurred, the latter suggesting the liklihood of an inhibitory influence from caudalis as well as the facilitatory influence. The effects of interactions of innocuous and noxious V stimuli were likewise subject to ascending influences from caudalis. Cold block also reversibly depressed responses to tooth pulp and V cutaneous noxious stimuli, although pulp-evoked responses were depressed less than mechanical or infraorbital nerve-elicited responses. Our results indicate that caudalis, as well as acting as a relay site to thalamus, also exerts a predominantly facilitatory influence on the relay to the thalamus and local reflex centres of mechanoreceptive and nociceptive information through the V main sensory-oralis region. The findings also might in part explain the analgesia, partial loss of tactile sensibility, and relief from V neuralgia reported after V tractotomy.