Ammonia and organic acids constitute a major part of the bacterial metabolites formed in carious decay. The aim of the present study was to investigate their effect on the intradental sensory nerves. Nerve impulse activity was recorded from canine teeth in cats after application of the test solutions in deep dentinal cavities. Ammonia (17-134 mM) consistently generated nerve impulses, whereas organic acids (0.001-1 M) failed to induce any impulse activity. In contrast, acid application resulted in an inhibition of the ongoing nerve activity induced by various stimuli (hypertonic NaCl solution, mechanical pulp exposure, and compound 48/80). However, acid treatment of the cavities resulted in an enhanced neural response to ammonia stimulation. Thus, the present results demonstrate that these bacterial metabolites can influence intradental sensory nerve activity. It is suggested that they may also modulate the symptoms from decayed teeth.