We examined the effects of increases in stimulation frequency on the intracellular potassium activity and cell volume of feline and canine Purkinje fibers and feline ventricular fibers. Intracellular potassium activity was measured with potassium liquid ion exchanger microelectrodes in single- and double-barrel configuration. Cell volume was measured with an electrophysiologic technique. From single-barrel electrode recordings, intracellular potassium activities (means +/- SE) in feline Purkinje fiber, feline papillary muscle, and canine Purkinje fiber are 117.1 +/- 5.4, 87.3 +/- 3.1, and 112.4 +/- 4.4 mM, respectively, with [K]o (external potassium concentration) of 5.4 mM. Thus direct evidence for a 30-mM gradient in intracellular potassium activity from Purkinje fiber to ventricular muscle is presented. Intracellular potassium activity did not change in any tissue studied during or after 3 min of 4 Hz stimulation. The lack of change is partially explained by a reversible 2% decrease in cell volume induced by the stimulation protocol. Net losses greater than 2% seen in earlier studies probably reflect either differences between perfused vs. superfused preparations or functional sequestration of cellular potassium involved in transfers during frequency changes.