In each of 6-amino-3-methyl-5-nitroso-2-(pyrrolidin-1-yl)pyri­m­idin-4(3H)-one monohydrate, C9H13N5O2·H2O, (I), and 6-amino-2-dimethyl­amino-3-methyl-5-nitroso­pyrimidin-4(3H)-one monohydrate, C7H11N5O2·H2O, (II), the inter­atomic distances indicate significant polarization of the electronic structures of the pyrimidinone mol­ecules. In each compound, the organic component contains an intra­molecular N—H⋯O hydrogen bond. The mol­ecular components in (I) are linked by a combination of two-centre O—H⋯O, O—H⋯N and N—H⋯O hydrogen bonds and a three-centre O—H⋯(NO) hydrogen bond to form a broad ribbon containing five distinct ring motifs. In compound (II), three inter­molecular hydrogen bonds, one each of the O—H⋯O, O—H⋯N and N—H⋯O types, link the mol­ecules into sheets containing equal numbers of centrosymmetric R44(10) and R108(34) rings.