Abstract 2,6(1H,3H)‐Oxazolo[5.4‐d]pyrimidinedione and 2,6(1H,3H)‐thiazolo[5.4‐d]‐pyrimidinedione are xanthine analogs related to oxazolo [5.4‐d]pyrimidine and thiazolo [5.4‐d] pyrimidine in the same way that xanthine, 2,6(1H, 3H)‐7‐imidazo‐[4.5‐d]pyrimidinedione, is related to purine, 7‐imidazo[4.5‐d]pyrimidine. (The numbering employed for xanthine derivatives has been retained in naming the analogs.) In the analogs, the 7‐imidazo ring of xanthine is replaced by an oxazole or thiazole ring. By substitution of hydrogens with methyl groups at positions 1 and 3 (as in theophylline or caffeine), or at position 1 only (as in theobromine), compounds analogous to the xanthines have been prepared. Related compounds substituted at position 8, are also described.