The consequences of one-electron oxidation and one-electron reduction were studied for 4-aminopyrimidine (4APM), which displays prototropic tautomerism. Since experimental techniques are incapable of detecting less than 0.1% of minor tautomers, quantum-chemical calculations [DFT(B3LYP)/6-311+G(d,p)] were carried out for all possible tautomers of neutral 4AMP and its redox forms, 4APM (+ •) and 4APM (- •). Four tautomers were considered: one amine and three imine tautomers (two NH and one CH form). Geometric isomerism of the exo = NH group was also taken into account. One-electron oxidation (4APM - e → 4APM (+ •)) has no significant effect on the tautomeric preferences; it influences solely the composition of the tautomeric mixture. The amine tautomer is favored for both 4APM (+ •) and 4APM. An interesting change in the tautomeric preference occurs for 4APM (- •). One-electron reduction (4APM + e → 4APM (- •)) favors the C5 atom for the labile proton. The preference of the imine CH tautomer in the tautomeric mixture of 4APM (- •) may partially explain the origin of CH tautomers in nucleobases.