Treatment of 6,7-dimethylpteridine with aqueous alkali and with aliphatic amines gave 7-(3,4-dihydro-6,7-di-methylpteridin-4-ylmethyl)-3,4-dihydro-4-hydroxy-6-methyl-(1) and 7-(6,7-dimethylpteridin-4-ylmethylene)-3,7-dihydro-6-methyl-pteridine (3a), respectively. 7-Methylpteridine and an amine similarly produced 3,7-di-hydro-7-(7-methylpteridin-4-ylmethylene)pteridine (3b). 6,7-Dimethylpteridine and hydroxylamine yielded 3-hydroxyiminomethylamino-5,6-dimethylpyrazine-2-carbal-dehyde oxime (9a). 2,6,7-Trimethylpteridine and methoxyamine afforded 2-(1 -methoxyimino)ethylamino-3-methoxyiminomethyl-5,6-dimethylpyrazine (9b). Malononitrile reacted with (a) 6,7-dimethyl- and 2,6,7-tri-methyl-, (b) 4-methyl-, and (c) 4,6,7-trimethyl-, and 2,4,6,7-tetramethyl-pteridine to give, respectively, (a) the 2,3-dimethyl (13)(from the first two). (b) the 8-methyl (15), and (c) the 2,3,8-trimethyl (16) derivatives of 6-amino-pyrido[2,3-b]pyrazine-7-carbonitrile. Cyanoacetamide and 6,7-dimethylpteridine yielded 6-amino-2,3-di-methylpyrido[2,3-b]pyrazine-7-carboxamide (14). Dimedone, malonamide, and methanol (the last named only under acidic conditions) added to 6,7-dimethylpteridine across the 3,4-double bond to give 4-substituted 3,4-dihydro-6,7-dimethylpteridines [respectively (18a and b) and (17)]. 2-Methylpteridine and dimedone gave 3,4-dihydro-2-methyl-4-(4.4-dimethyl-2,6-dioxocyclo-hexyl)pteridine (18c). With ethyl acetoacetate, 2-methyl- and 4-methyl-pteridine yielded 2-methyl (19a) and 4-methyl derivatives of ethyl 5,5a,8a.9-tetrahydro-7-methylfuro[2,3-g]pteridine-8-carboxylate (19b), respectively. 4-Methylpteridine added two molecules of benzylamine across the 5,6- and 7,8-double bonds to give 6,7-bis-(benzylamino)-5,6,7,8-tetrahydro-4-methylpteridine (21 ). Consideration of the u.v. spectra of the dimedone adducts (18a, c, and d) revealed that the neutral species were zwitterionic. U.v. and 1H n.m.r. spectra are reported and discussed.