Reaction of 2-(methyl-sulfan-yl)pyrimidin-4-amine with the 5-(methoxy-vinyl-idene) derivative of Meldrum's acid and subsequent heating of the product in Dowtherm fluid yielded the title compound, C(8)H(7)N(3)OS, which was proven to contain a bicyclic 4H-pyrimido[1,6-a]pyrimidine system. All non-H atoms of the mol-ecule are coplanar within 0.15 Å. The bond-length distribution in the bicyclic core shows localization of the double bonds. The geometry of the intra-molecular S⋯O 1,5-contact [2.534 (2) Å] is consistent with the existence of an attractive inter-action.