The effect of radial artery cannulation on local tissue oxygenation and acid-base balance was studied in 20 patients. Using capillary samples drawn from each thumb, a significant difference was observed between capillary PO2 on each side (p less than 0.001), with the higher PO2 on the cannulated side. There were no significant differences between sides in terms of [H+] or PCO2. It is considered that the differences in capillary PO2 were due to local tissue vasodilatation. The presence of chlorbutol in the sodium heparin flushing solution is implicated as the causative factor. Further study of 20 patients using preservative-free heparin demonstrated no significant differences between cannulated and non-cannulated sides in terms of capillary PO2, PCO2 or [H+]. Sodium heparin with chlorbutol appears to improve local tissue oxygenation and is recommended for use in flushing solutions for invasive arterial monitoring.