(also called pyrimidine), any of a group of naturally occurring compounds that are derivatives of the heterocyclic nitrogenous base pyrimidine. Pyrimidine bases, which are components of nucleic acids, are very important in metabolism. Nucleic acids contain large amounts of the pyrimidine bases cytosine, or 2-hydroxy-6-aminopyrimidine; uracil, or 2,6-dihydroxypyrimidine; and thymine, or 5-methyluracil. They also contain smaller amounts of minor bases, for example, 5-methylcytosine. The number of minor bases in transfer ribonucleic acid (RNA) is especially high.

The combination of certain carbohydrates with purine or pyrimidine bases is called a nucleoside. The nucleosides in RNA consist of pyrimidine bases bound by a glycosidic linkage to the carbohydrate ribose, while those in deoxyribonucleic acids (DNA) consist of pyrimidine bases bound to deoxyribose. Nucleotides, which are the monophosphate esters of nucleosides, are the basic structural units of nucleic acids. In DNA, the pyrimidine-base content equals that of the purine bases, while RNA usually contains somewhat fewer pyrimidine bases than purine bases (the relative amount of nitrogenous bases in a nucleic acid is described by the Chargaff rule). As a result of their capacity to interact specifically with purine bases according to the complementarity principle, pyrimidine bases participate in the coding and transfer of the genetic information that is contained in nucleic acids.

Nucleotides that contain pyrimidine bases are also important in metabolism, for example, uridine diphosphate in carbohydrate metabolism and cytidine diphosphate in lecithin metabolism. Pyrimidine bases are synthesized in the cell from derivatives of orotic acid.