The diving response is initiated by apnea and facial immersion in cold water and includes, besides bradycardia, peripheral vasoconstriction, while cerebral perfusion may be enhanced. This study evaluated whether facial immersion in 10 degrees C water has an independent influence on cerebral perfusion evaluated as the middle cerebral artery mean flow velocity (MCA V(mean)) during exercise in nine male subjects. At rest, a breath hold of maximum duration increased the arterial carbon dioxide tension (Pa(CO(2))) from 4.2 to 6.7 kPa and MCA V(mean) from 37 to 103 cm/s (mean; approximately 178%; P <0.001). Similarly, during 100-W exercise, a breath hold increased Pa(CO(2)) from 5.9 to 8.2 kPa (P <0.001) and MCA V(mean) from 55 to 113 cm/s ( approximately 105%), and facial immersion further increased MCA V(mean) to 122 cm/s ( approximately 88%; both P <0.001). MCA V(mean) also increased during 180-W exercise (from 47 to 53 cm/s), and this increment became larger with facial immersion (76 cm/s, approximately 62%; P <0.001), although Pa(CO(2)) did not significantly change. These results indicate that a breath hold diverts blood toward the brain with a >100% increase in MCA V(mean), largely because Pa(CO(2)) increases, but the increase in MCA V(mean) becomes larger when combined with facial immersion in cold water independent of Pa(CO(2)).