Although pain and dyspnoea are common symptoms in pleural diseases, there are few studies on the sensory innervation of the pleura. After removal of all muscles in a studied intercostal space, we investigated in rabbits the afferents in internal intercostal nerve by applying on the internal thoracic wall pieces of gauze soaked in warmed (37°C) buffered saline (mechanical stimulation) or solutions containing lactic acid, inflammatory mediators or capsaicin (chemical stimulation). The afferent conduction velocity ranged from 0.5 to 14 m.s-1. Most units (97%) were activated by mechanical stimulation of the pleura (local positive pressure range = 4.5 to 8.5 cmH2O) and we found a linear relationship between the discharge rate of afferents and the force applied to the thoracic wall. The majority of mechanosensitive units (70%) also responded to one or several chemical agents. Thus, the afferents were activated by lactic acid (49%) and/or a mixture of inflammatory mediators (50%). Local application of capsaicin elicited an initial increased or decreased background afferent activity in 57% of the afferents, a delayed decrease in firing rate being noted in some units initially activated by capsaicin. Capsaicin blocked the afferent response to a further application of inflammatory mediators but did not affect the mechanosensitive units. Thus, sensory endings connected with thin myelinated and unmyelinated fibres in the internal intercostal nerve detect the mechanical and chemical events present in pleural diseases.