Capillary fragments were isolated from guinea pig hearts, and their electrical properties were studied using the perforated-patch and cell-attached mode of the patch-clamp technique. A voltage-dependent K⁺ current was discovered that was activated at potentials positive to 20 mV and showed a sigmoid rising phase. For depolarizing voltage steps from 128 to +52 mV, the time to peak was 71 ± 5 ms (mean ± SE) and the amplitude of the current was 3.7 ± 0.5 pA/pF in the presence of 5 mM external K⁺. The time course of inactivation was exponential with a time constant of 7.2 ± 0.5 s at +52 mV. The current was blocked by tetraethylammonium (inhibitory constant ~3 mM) but was not affected by charybdotoxin (1 µM) or apamin (1 µM). In the cell-attached mode, depolarization-activated single-channel currents were found that inactivated completely within 30 s; the single-channel conductance was 12.3 ± 2.4 pS. The depolarization-activated K⁺ current described here may play a role in membrane potential oscillations of the endothelium.