The dielectric breakdown in the membranes of cells of Valonia utricularis was investigated using intracellular electrodes and 500 mu sec current pulses. Electrical breakdown, which occurs when the membrane potential reaches a well-defined critical value, is not associated with global damage to the cell or its membranes (the membrane reseals in smaller than 5 sec). It was thus possible to investigate the effect of temperature on dielectric breakdown in single cells. It was found that the critical potential for breakdown was strongly dependent on temperature, decreasing from similar to 100 mV at 4 degrees C to similar to 640 mV at 30 degrees C. The decrease in the breakdown potential with increasing temperature and the very short rise-time of the breakdown current (similar to 1mu sec) suggests that the Wien field dissociation does not play a major role in the breakdown process. It is shown that the nonlinear I-V characteristics observed at different temperatures can be accurately accounted for with no adjustable parameters, by considerations of the mechanical compression of the membrane due to stresses induced by the electric field. Electrical breakdown on this scheme results from an electromechanical instability in the membrane. On this basis the present results indicate that the elastic modulus of the region of the membrane where breakdown occurs, decreases by a factor of 2 with increasing temperature from 4 to 30 degrees C. On the assumption of a thickness of 4.0 nm and a dielectric constant of 5, the elastic modulus is estimated to have a value of 5 times 10-6Nm- minus 2 at 20 degrees C.