2-Methylhypoxanthine-% thiol has been condensed with chloroacetic and p-chloropropionic acids to give the corresponding thioacetic and thiopropionic acids which on cyclization gave 4hydroxy-2-methyl-6, 7-dihydrothiazolo-[2.3-f] purine-6-one and 4hydroxy-2-methyl-6, 7-