. COX-2 inhibitors have been successful in treating inflammatory diseases like acute pain, rheumatoid arthritis and osteoarthritis; a few of them are also being studied for treating different types of cancer and Alzheimer's disease (1). Despite a few recent cautionary reports, the coxib treatment has a high degree of benefit over risk, and strategies for using NSAIDs have been described by Antman et al. (2). A series of 3,4-diaryl-1,2,5-oxadiazoles and 3,4-diaryl--1,2,5-oxadiazole N-oxides were prepared and evaluated for COX-2 and COX-1 binding affinity in vitro and for anti--inflammatory activity by the rat paw edema method. p--Methoxy (p-OMe) substituted compounds 9, 21, 34, 41, 42 showed COX-2 enzyme inhibition higher than that showed by compounds with other substituents. 3,4-Di(4--methoxyphenyl)-1,2,5-oxadiazole N-oxide (42) showed COX-2 enzyme inhibition of 54% at 22 µmol L -1 and COX-1 enzyme inhibition of 44% at 88 mmol L -1 concentrations, but showed very low in vivo anti-inflammatory activity. Its deoxygenated derivative (21) showed lower COX-2 enzyme inhibition (26% at 22 µmol L -1 ) and higher COX-1 enzyme inhibition (53% at 88 µmol L -1 ) but, marked in vivo anti-inflammatory activity (71% at 25 mg kg -1 ) vs. celecoxib (48% at 12.5 mg kg -1 ). Molecular modeling (docking) studies showed that the methoxy group is positioned in the vicinity of COX-2 secondary pocket and it also participates in hydrogen bonding interactions in the COX-2 active site. These preliminary studies suggest that p-methoxy (p-OMe) group in one of benzene rings may give potentially active leads in this series of oxadiazole/N-oxides.