The antinociceptive effects of the S(+)-enantiomer of flurbiprofen (potent inhibitor of cyclooxygenase) and the R(-)-enantiomer (500 times less potent) were investigated in the spinal cord of 20 anesthetized rats. In lumbar segments, 20 wide-dynamic-range dorsal horn neurons with knee joint input were recorded extracellularly. After induction of an acute inflammation in the knee joint by kaolin and carrageenan, the neurons developed hyperexcitability consisting of enhanced responses to stimuli applied to the inflamed knee and the noninflamed ankle and an expansion of receptive fields. Intravenous administration of R(-)-flurbiprofen (1-9 mg/kg) and S(+)-flurbiprofen (0.3-9 mg/kg) at 5.5 to 8.5 h after kaolin, dose dependently reduced the neurons' responses to pressure applied to the inflamed knee (18 of 18 neurons) and the noninflamed ankle (17 of 17 neurons) and paw (8 of 8 neurons). R(-)-flurbiprofen decreased the receptive field size in 8 of 16 neurons, S(+)-flurbiprofen in 10 of 16 neurons. The suppressive effects started 3 to 6 min and reached a maximum 9 to 15 min after i.v. administration. S(+)-flurbiprofen was more potent than the R(-)-enantiomer. When injected directly into the knee joint, S(+)-flurbiprofen (50 and 80 micrograms), but not the R(-)-enantiomer (100 and 180 micrograms) reduced the hyperexcitability in 12 of 12 neurons. These results suggest a central site of antinociceptive action for R(-)-flurbiprofen and S(+)-flurbiprofen and an additional peripheral site for S(+)-flurbiprofen. The doses used in these experiments did not produce any sedative effects in rats subjected to behavioral testing.