Many of the nonsteroidal anti-inflammatory drugs (NSAIDs) are marketed as racemic mixtures, composed of (R)- and (S)- enantiomers. Racemic NSAIDs are potent cyclooxygenase (COX) inhibitors only through the action of the (S)- enantiomers, as the (R)- enantiomers do not exhibit COX inhibition. However, the (R)- enantiomer of ketoprofen exhibits potent analgesic activity and minimal ulcerogenic potential. To extend these observations, we examined the (R)- and (S)- enantiomers of RS- ketorolac, (S)- ketorolac exhibited potent COX1 and COX2 enzyme inhibition, whereas (R)- ketorolac was > 100-fold less active on both COX subtypes. Both enantiomers did not affect norepinephrine or serotonin uptake sites, and nitric oxidase or lipoxygenase activities, nor did they demonstrate any affinity for opioid receptors (mu, delta, or kappa). In experimental models, (S)- ketorolac exhibited about 10-fold greater activity than (R)- ketorolac in the murine phenylquinone writhing model. In this model, morphine sulfate was effective at much lower doses, however, and neither (R)- nor (S)- ketorolac showed any morphine-sparing effect. In the rat gait test for analgesia in the foot paw after injection of brewers yeast suspension, neither (R)- nor (S)- ketorolac affected paw volume. However, both provoked changes in gait scores, the (S)- enantiomer being 30-fold more potent than the (R)- enantiomer. A similar reduction was observed with respect to ulcerogenic potential, measured by direct microscopic changes after test conclusion. These findings suggest that (R)- ketorolac may possess analgesic activity that is independent of COX inhibition and may be associated with reduced ulcerogenic potential compared to effects exhibited by (S)- ketorolac.