The kinetics of H+ and K+ activities were recorded during and after direct electrical activation of the brain cortex (cat). H+ activity was measured with H+-sensitive glass microelectrodes (tip diameters of 1--4 micron) and K+ activity was registered with double-barrelled ion-sensitive microelectrodes (tip diameters of 1--3 micron). It could be shown that extracellular H+ activity initially decreased for a few seconds and increased only after the 7.s. Maximum acidosis was always noticed after stimulation ended. Alkalotic as well as acidotic changes were the higher the stronger the stimulation parameters were. K+ activity increased very rapidly after stimulation began, reached its maximum when stimulation ended and then decreased to its initial value with an undershoot. It is concluded that the functional hyperemia of microflow could be triggered by the rapid increase in K+ activity, whereas the initial alkalotic change of extracellular pH means that H+ activity does not play a role in the first phase of this kind of hyperemia. The alkalotic shift is interpreted to be caused by the washout of C02 due to the rapid increase in microflow. In the further course, H+ activity obviously contributes to the maintenance of functional hyperemia. In this later period K+ activity is always below the control value.