The antinociceptive effect of spinally delivered S- and R-flurbiprofen, enantiomers of 2-arylpropionic acid, was studied in rats using the formalin test. Intrathecal injections of S- or R-flurbiprofen produced a significant reduction of the second phase of the formalin test, with no effect on the first phase. The maximal suppression of the second phase was similar for both agents (about 50% at the highest doses). While both agents were active, S-flurbiprofen was significantly more potent than R-flurbiprofen. The potency ratio between the dose-response curves was 10 (6-20; 95% confidence intervals). These results demonstrate that both S- and R-flurbiprofen produce antinociception after spinal administration. The potency difference is similar to that for inhibition of cyclooxygenase in brain tissue and supports the hypothesis that cyclooxygenase products are involved in prolonged spinal nociceptive transmission.