The release of both radioactive and endogenous purines was investigated in rat brain cortical, hippocampal and striatal slices at rest and following stimulation with electrical fields. Purines were labelled by incubating the slices with 3H-adenine. The purine efflux at rest and that evoked by electrical stimulation (10 Hz. 5 min) was analyzed by HPLC with ultraviolet absorbance detection. Both radioactive and endogenous purines in the effluent consisted mainly of hypoxanthine, xanthine, inosine and adenosine. No qualitative differences in the composition of the released purines were found in the three areas investigated. Electrical stimulation evoked a net increase in both radioactive and endogenous purine release. However the increase in 3H-adenosine following electrical stimulation was twice as large as that of endogenous adenosine. The electrically evoked release of both radioactive and endogenous purines was greatest in hippocampal slices and progressively smaller in cortical and striatal slices. In the three areas the addition of 0.5 microM tetrodotoxin to the superfusing Krebs solution brought about a similar (83-100%) reduction in evoked 3H-purine and endogenous purine release. Superfusion of the slices with calcium-free Krebs solution containing 0.5 mM EGTA reduced evoked release of 3H-purines by 58-60% and that of endogenous purine components by 54-89%. The results demonstrate similar characteristics for both radioactive and endogenous purine release but indicate that the most recently synthetized adenosine is the most readily available for release. The features of the electrically evoked purine release support a neuronal origin of adenosine and derivatives and are consistent with the hypothesis of discrete regional differences in adenosine neuromodulation.