In patches from membranes containing many active channels it is difficult to measure the voltage dependence of activation. We have developed a technique using ramp potentials to measure activation rapidly over a wide range of potentials. If the rate of change of holding potential is small compared to the activation or deactivation time constant of the channels voltage ramps can be used to measure N*p(open), where N is the number of channels per patch and p(open) is the open probability of the single channel. This provides a continuous reading of N*p(open) and allows an efficient measurement of channel activation over large voltage ranges. The voltage dependent activation of 200 pS Ca2(+)-activated K channels in excised patches from freshly dispersed canine colonic and gastric myocytes studied with voltage ramps was in good agreement with data obtained and analyzed with conventional methods at selected stationary voltages. This method has been found to be useful for studies of patches containing large number of channels and may be applied to characterize the voltage dependent activation of channels in several other tissues.