To investigate the effect of the level of baseline cerebral blood flow (CBF) on local CBF augmentation by activation, we have used positron emission tomography to measure regional CBF (rCBF) in 12 normal volunteers with and without photic stimulation during hypocapnia, normocapnia, and hypercapnia. The increase in rCBF in the primary visual cortex by photic stimulation was 10.8 +/- 3.1, 18.6 +/- 9.3, and 19.5 +/- 6.1 ml 100 ml-1 min-1 in hypo-, normo-, and hypercapnia, respectively. The increase was significantly smaller in hypocapnia than in normocapnia (p < 0.005). The fractional CBF increase caused by the photic stimulation was the same in all breathing conditions. This result indicates that the magnitude of the CBF increase induced by neuronal activity correlates proportionally with the level of baseline CBF.