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The inhibiting influences of some non-steroidal anti-inflammatory drugs (NSAIDs) and other agents on cyclooxygenase (CO) and lipoxygenase (LO) activities in some enzyme preparations were investigated. Washed rat platelets (CO and 12-LO), rat polymorphonuclear leukocytes (PMNs, CO and 5-LO), rat renal medulla homogenate (CO), and purified soybean LO (15-LO) were used as enzyme preparations. The IC50 values of drugs on the enzyme activities were determined in each preparation. In addition, the inhibitory activities of the drugs on the generation of chemiluminescence from PMNs stimulated by phorbol myristate acetate were tested. NSAIDs (indomethacin, ketoprofen and phenylbutazone) showed a selective inhibition of CO in each preparation, but benoxaprofen inhibited both enzymes, especially in PMNs. BW755C, 1-phenyl-3-pyrazolidone (phenidone), toluene-3,4-dithiol (dithiol), 6-ethoxy-1,2-dihydro-2,2,4-trimethylquinoline (ethoxyquin) and acetone phenylhydrazone (APH) inhibited both enzyme activities. Nordihydroguaiaretic acid (NDGA) showed a relatively selective inhibition of LO in all the preparations used. APH inhibited soybean 15-LO markedly compared with the other enzymes tested. Ethoxyquin inhibited COs more markedly than LOs, and with regard to LO inhibition, it inhibited 5-LO in PMNs markedly. BW755C, phenidone, ethoxyquin, NDGA, APH and dithiol, which strongly inhibited 5-LO, showed an inhibitory activity on the generation of chemiluminescence from PMNs activated with phorbol ester.