The effects of tooth surface stimulation on intradental nerve activity (INA) and subsequent pain perception were studied in human lower incisors later to be extracted for periodontal reasons. The INA elicited by hot gutta-percha briefly applied to the tooth surface was monitored by means of labial electrodes deeply implanted in the dentin and perceived pain was continuously rated using a finger-span technique. After each stimulation the subject was also requested to select a sensory descriptor that was appropriate to describe the maximal pain intensity. The very first application of heat induced a typical pattern of nerve activity consisting of three phases. An initial burst of 3-5 s duration, phase I, was followed by a depression relative to the baseline lasting for 20-30 s, phase II, that gradually turned into phase III constituting a slowly increasing firing rate. The spontaneously emerging activity (phase III) in the absence of a physical stimulus passed unnoticed by all the subjects despite an average increase in firing rate of 67% relative to the prestimulus noise level. Repeated heat applications at short intervals led to a decrease and finally to abolishment of the whole nerve response. The lack of pain during phase III may be explained in two ways: the rate of increase in firing frequency may be too slow to trigger those perceptual pathways involved in processing of pain; the slow development of the increased sensory nerve activity may lead to central habituation.(ABSTRACT TRUNCATED AT 250 WORDS)