The free extracellular concentrations of acyclovir and (R)-9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine (H2G) in the blood and in brain tissue of rats were measured in microdialysis samples by high-pressure liquid chromatography with UV detection. The half-life of acyclovir in blood was 2 h, while that of H2G was 28 min. The concentration attained in the brain compared with the concentration in blood was slightly lower for H2G than for acyclovir. The results show that the pharmacokinetic profiles of acyclic nucleosides may vary as a consequence of the structure of the acyclic chain corresponding to the sugar moiety.