We report the preparation of 3-amino-4,5-dihydro-1,2,4-thiadiazole 1,1-dioxide and of its 4-methyl derivative which are of interest as potential analogues of creatinine. The thiadiazoles are obtained from chloromethylsulfonylation ofS-benzylisothiourea, followed by cyclization of the chloromethanesulfonamide under basic conditions in the presence of 2-(t-butoxycarbonyloxyimino)-2-phenylacetonitrile or of methyl iodide.