In order to examine analgesic and antiinflammatory activities of the position isomers of 4-ethoxy-2-methyl-5-morpholino-3 (2H)-pyridazinone (1, emorfazone), an analgesic-antiinflammatory drug, 5-ethoxy-2-methyl-4-morpholino-3 (2H)-pyridazinone (2), 6-ethoxy-2-methyl-4-morpholino-3 (2H)-pyridazinone (3) and 6-ethoxy-2-methyl-5-morpholino-3 (2H)-pyridazinone (4) were prepared. Since 4 showed the most strong activity among the compounds tested, various 6-alkoxy- or 2-cyclohexyl- or 2-phenyl-5-substituted amino-3 (2H)-pyridazinones (15, 16) were prepared and examined for their activities. As a result, 4 and 2-methyl-5-morpholino-6-n-propoxy-or 6-n-butoxy-3 (2H)-pyridazinone (15b, c) and 6-ethoxy-2-ethyl-5-morpholino-3 (2H)-pyridazinone (151) were revealed to be more potent in analgesic and antipyretic activities than commercial drugs (1, aminopyrine, mepirizole, tiaramide HCl, phenylbutazone, mefenamic acid). On the basis of the available data, the structure-activity relationship in a series of 6-alkoxy-2-alkyl-5-substituted amino-3 (2H)-pyridazinones (15, 16) was also discussed.