Although direct-current (DC) electrical shocks do not significantly alter left ventricular systolic performance, their effects on ventricular diastolic properties and the role of sympathetic stimulation in maintaining postshock contractility are unknown. Accordingly, we studied both diastolic and systolic left ventricular performance in seven open-chest dogs before and after three consecutive 50 joule DC shocks applied directly to the heart. High-fidelity left ventricular pressure and segmental dimensions were recorded as pressure was varied systematically under conditions of beta-adrenergic blockade. The exponential left ventricular end-diastolic pressure-segment length relationship was not significantly altered by electrical shocks. Likewise, pre- and postshock beats matched for loading conditions did not differ in peak positive dP/dt, end-systolic segment length, or percentage of segment shortening, although mean normalized shortening rate decreased by approximately 11% (p less than 0.05). However, the time constant (T) for isovolumic pressure fall, an index of the rate of left ventricular relaxation, increased after shocks in all animals (T = 42.2 +/- 5.3 and 51.1 +/- 7.0 msec before and after shock, respectively; (p less than 0.01). We conclude that high-energy electrical shocks applied directly to the heart cause modest slowing of left ventricular relaxation but do not have important effects on left ventricular compliance or on systolic myocardial performance. These data suggest that DC shocks have only minor functional consequences when applied to normal hearts, even when sympathetic compensation is prevented.