Novel series of pyrazolo [5, 1-b] 1, 3-oxazolidines, pyrazolo [5, 1-b] 1, 3-oxazines and imidazolidino [1, 2-d] pyrazoles were synthesized. These compounds were evaluated in vitro for their ability to inhibit cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2) in human whole blood (HWB). Several of the compounds were found to be novel and selective COX-2 inhibitors, the most potent and selective being 1-(5-cyclohexyl (2H, 3H-pyrazolo [5, ...