When Ca2+ was omitted from the physiologic superfusing solution or when 3.0 mmol/l EGTA was added, cells in 20 canine right ventricular false tendons depolarized from -84±5 mV to -48±9 mV (mean ± 1 s.d.). A similar response (from -83±5 mV to -43±9 mV) omission followed of Mg2+ alone or addition of 3.0 mmol/l calcium EDTA. In all trials the degree of depolarization reached a stable level within 30 min. Omission of both Ca2+ and Mg2+ from the superfusing solution resulted in depolarization from -83±5 mV to -38±11 mV within 19 min. Complete recovery of transmembrane potential from each of these several manipulations was seen within 30 min after resuming superfusion with control solution. In contrast to these observations, addition of 3.0 mmol/l disodium EDTA brought about depolarization from -84±5 mV to -1±5 mV within 5 min. Further-more, superfusion with disodium EDTA for 30 min resulted in permanently depolarized false tendon cells. Studies with electron microscopy demonstrated that La3+ was able to cross the sarcolemma of these cells. Thus, the removal of either Ca2+ or Mg2+ alone, or omission of both, produces a consitent but readily reversible depression of electrical activity. On the other hand, the chelation of both divalent cations causes more rapid and complete depolarization within 5 min and which was not reversible after 30 min. Physiological irreversibility was associated with abnormal permeability of the cells by lanthanum. Electrical stability in canine false tendon cells appears to be dependent on ready availability of both Ca2+ and Mg2+. © 1979.