A C-fibre reflex elicited by electrical stimulation within the territory of the sural nerve was recorded from the ipsilateral biceps femoris muscle in anaesthetised rats. Such reflex responses can be inhibited by applying noxious conditioning stimuli to heterotopic areas of the body. These inhibitory processes have been termed diffuse noxious inhibitory controls. The responses were recorded before, during and after the immersion of the tail in a thermoregulated waterbath (at 50 degrees C) for 1 min. The C-fibre reflex responses were depressed by a maximum of 71 +/- 3% at 45 s after the start of such conditioning stimuli. A dose of 3 mu g/kg buprenorphine completely blocked the inhibition and post-stimulus effects triggered by the heterotopic noxious stimuli. In the 0.3-3 mu g/kg range, buprenorphine increased, in a dose-dependent manner, the magnitude of the inhibition. These doses did not produce any changes in the C-fibre reflex itself. The results are discussed in terms of the mechanisms underlying the analgesic properties of buprenorphine.