Patients under antiulcer therapy may suffer from a concurrent disease which requires the use of one of NSAIDs, In such cases, these patients are likely to receive a combination of one of the antiulcer drugs plus the NSAIDs. Accordingly, the simultaneous use of one of the NSAIDs with one of the antiulcer drugs may lead to drug-drug interaction. The present work was devoted to the assessment of the modification of the anti-inflammatory, antipyretic and analgesic effects of celecoxib after the concurrent administration of the antiulcer drugs, omeprazole and ranitidine. This study was performed on two animal species, rats and mice. Rats were used to detect the anti-inflammatory and the antipyretic activities of the investigated drugs. Mice were utilized to study the analgesic activity of the same drugs. Male albino rats were divided into 2 main groups: Group I. rats were divided into 5 subgroups, to study the anti-inflammatory activity of the tested drugs by induction of inflammation by subcutaneous injection of 0.1 ml of 20% brewer's yeast suspension into planter surface of the right hid paw and measuring the rats paw thickness according to the treatment received. Group II: rats were divided into 5 subgroups, to study the anti-pyretic activity of the tested drugs by induction of pyrexia by subcutaneous injection of 2.5 ml of 20% aqueous suspension of yeast dorsally and ventrally and recording the rectal temperature according to the treatment received. GroupIII: mice were divided into two set of experiment each of which divided into 5 subgroups. The analgesic activity of the drugs was evaluated by chemical method [p-benzoquinon induced-writhing response …