Abstract: The metabolic fate of guanine and of guanine ribonucleotides (GuRNs) in cultured rat neurons was studied using labeled guanine. 8‐Aminoguanosine (8‐AGuo), an inhibitor of purine nucleoside phosphorylase, was used to clarify the pathways of GMP degradation, and mycophenolic acid, an inhibitor of IMP dehydrogenase, was used to assess the flux from IMP to GMP and, indirectly, the activity of the guanine nucleotide cycle (GMP → IMP → XMP → GMP). The main metabolic fate of guanine in the neurons was deamination to xanthine, but significant incorporation of guanine into GuRNs, at a rate of ∼8.5–13.1% of that of the deamination, was also demonstrated. The turnover rate of GuRNs was fast (loss of 80% of the radioactivity of the prelabeled pool in 22 h), reflecting synthesis of nucleic acids (32.8% of the loss in radioactivity) and degradation to xanthine, guanine, hypoxanthine, guanosine, and inosine (49.3, 4.3, 4.1, 1.1, and 0.5% of the loss, respectively). Of the radioactivity in GuRNs, 7.9% was shifted to adenine nucleotides. The accumulation of label in xanthine indicates (in the absence of xanthine oxidase) that the main degradative pathway from GMP is that to xanthine through guanosine and guanine. The use of 8‐AGuo confirmed this pathway but indicated the operation of an additional, relatively slower degradative pathway, that from GMP through IMP to inosine and hypoxanthine. Hypoxanthine was incorporated mainly into adenine nucleotide (91.5%), but a significant proportion (6%) was found in GuRNs. The incorporation of hypoxanthine into GuRNs and of guanine into IMP (and inosine) indicates that the two arms of the guanine nucleotide cycle are active, but the actual rate of activity of this cycle and its metabolic importance are yet uncertain.