Pteridine reacted with various Michael reagents to give three kinds of products. (i) Dimedone, barbituric acid, thiobarbituric acid, diethyl malonate, and ethyl benzoylacetate added across the 3,4-double bond to yield 4-substituted 3,4-dihydropteridines (1). (ii) Benzyl (and ethyl) acetoacetate, and acetylacetone produced 8-substituted 5,5a,8a,9-tetrahydro-7-methylfuro[2,3-g]pteridines (2); all were formed by simple addition of one molecule of the Michael reagent across the 5,6- and 7,8-bonds of pteridine. (iii) Malononitrile and cyanoacetamide gave pyrido[2,3-b]pyrazines (4) by addition across the 3,4-bond followed by scission of that bond and a nitrogen-eliminating recyclisation. Explanations of this diverse behaviour are suggested. U.v. and 1H n.m.r. spectra are reported and discussed.