The application of negative pressure of -40 kPa (-300 mm Hg) for 10 min under a suction electrode placed on the surface of the spontaneously beating frog ventricle changed the extracellular potassium activity (aoK+) in three phases: a phase of rapidly rising aoK+, a slowly decaying phase and a phase of slowly rising aoK+. The changes reversed when suction was discontinued during the rapidly rising and decaying phase, but were not reversed during the slowly rising phase. Calcium extra-cellular activity (aoCa2+) decreased between -5 and -13 kPa, but increased during the higher pressures. The results indicated that injury of the ventricular tissue caused by a suction electrode elevated aoK+ and altered aoCa2+.