This study was undertaken to clarify whether stellate ganglion block (SGB) changed cerebral blood flow (CBF) in both atmospheric conditions (rest) and hyperbaric oxygen (HBO, under 2 atm absolute). Unilateral SGB was performed on 16 patients with sudden deafness with the injection of 8 ml of 1% mepivacaine and CBF was measured 20 min thereafter. The mean hemispheric blood flow on the block side (CBFi) and the nonblock side (CBFc) was measured by the intravenous xenon-133 method, being expressed as the initial slope index. In the atmospheric condition, the CBFi of seven patients before and after SGB was 50.4 +/- 8.4 and 49.0 +/- 5.9 (mean +/- SD) ml/100 g/min, respectively, with no significant change between them. There was also no significant change in the regional distribution of CBF, i.e., the blood flow of the frontal, pariental, temporal, and occipital lobes in hemispheres. In the hyperbaric condition, the CBFi of nine patients under HBO(mean PaO2 = 1,141 mm Hg) and HBO with SGB (mean PaO2 = 1,138 mm Hg) was 39.4 +/- 6.2 and 37.7 +/- 7.0 ml/100 g/min, respectively, being significantly less (p <0.05) than 46.5 +/- 8.0 ml/100 g/min at rest (mean PaO2 = 88 mm Hg). However, SGB had no effect on the reduction in CBFi in the hyperoxemic state. There was also no effect of SGB on CBFc, which was almost equal to the corresponding CBFi in the atmospheric and hyperbaric environments. It is concluded that unilateral SGB does not alter CBF and its regional distribution at least in neurologically normal humans, whether in the normoxemic or hyperoxemic state. It is also suggested that blockade of the cervical sympathetic nervous system is not preventive of the CBF reduction in the hyperoxemic state.