The effects of internal Na on the outflow of K ions through a certain type of Ca-dependent K channels of chromaffin cells (Marty, 1981) were studied in isolated patches. Application of 20 mM Na ions to inside-out patches (Hamill, Marty, Neher, Sakamann and Sigworth, 1981) on the cytoplasmic (called hereafter internal) side of the membrane induced short interruptions of the single channel currents. This effect was voltage dependent. In addition, internal Na ions reduced the opening probability of the channels. Outside-out patch recordings were obtained with 20 mM internal Na and varying external K concentrations. The blocking effects of internal Na ions were relieved by increasing the external K concentration.