Purine and pyrimidine base and nucleoside levels were measured in adult rabbit cisternal CSF and plasma by reversed-phase high-performance liquid chromatography. The concentrations of bases, nucleosides, and nucleoside phosphates were similar in plasma and CSF except for the adenosine phosphates and uracil which were higher in the plasma. In plasma and CSF, adenosine levels were low (0.12 microM) and guanosine, deoxyadenosine, deoxyguanosine, and deoxyinosine were not detectable (less than 0.1 microM); inosine and xanthine concentrations were 1-2 microM and hypoxanthine concentrations were approximately 5 microM; uridine (approximately 8 microM), cytidine (2-3 microM), and thymidine, deoxyuridine, and deoxycytidine (0.5-1.4 microM) were easily detectable. In both plasma and CSF, guanine, and thymine were undetectable (less than 0.1 microM), adenine and cytosine were less than 0.2 microM, but uracil was present (greater than 1 microM). Adenosine, inosine, and guanosine phosphates were also detectable at low concentrations in CSF and plasma. These results are consistent with the hypothesis that purine deoxyribonucleosides are synthesized in situ in the adult rabbit brain. In contrast, pyrimidine deoxyribonucleosides and ribonucleosides, and purine and pyrimidine bases are available in the CSF for use by the brain.