pyrimidine, type of organic base found in certain coenzymes and in the nucleic acids of plant and animal tissue. The three major pyrimidines of almost universal distribution in living systems are cytosine, thymine, and uracil.

Any of a class of heterocyclic compounds with a ring structure of four carbon and two nitrogen atoms. The simplest member, pyrimidine itself (C₄H₄N₂), is not common, but derivatives with the structure are. Examples include thiamine (vitamin B₁), several sulfa drugs, barbiturates, and three of the bases in nucleic acids (cytosine, thymine, and uracil).

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