This study was carried out to determine both the effect of systemic paracetamol on the C-fibre evoked reflex activity, a test sensitive to centrally acting analgesic drugs, and the influence of an intrathecally administered 5HT3 receptor antagonist, tropisetron. Paracetamol (200, 300, 400 mg kg-1, i.v.) dose-dependently decreased (maximal effects -60 +/- 8%) the C-evoked responses for a duration of 90 min (for the lowest dose). This effect was totally suppressed by tropisetron (1 microgram, i.t.). These data confirm previous studies suggesting a central effect of this drug and demonstrate the involvement of a spinal 5HT3 mediated serotonergic mechanism.