A series of novel 5-sulfoxide-substituted pyrazolo[5,1-d][1,2,3,5]tetrazin-4(3H)ones 4a-j were designed and efficiently synthesized via a diazotization of 5-amine-3-methylsulfinyl1H-pyrazole, followed by cycloaddition with aryl isocyanate. A possible reaction mechanism is outlined and discussed. These new compounds exhibit some biological activity as preliminary bioassay indicated. Their structures were confirmed with1 H NMR, IR and elemental analysis.