Patch-clamp recordings were used to study the activation of ion channels in the cell membrane of cultured embryonic chick myotubes treated with the specific activator of protein kinase C, the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA; 1 x 10(-7) M). Myotubes exhibited a spontaneous channel activity when the TPA-induced dedifferentiative processes developed. This consisted in the activation of inward current channels (approximately 35 pS conductance; approximately 6 ms open time). These spontaneously active channels were insensitive to alpha-bungarotoxin, curare, atropine and tetrodotoxin and were not inhibited by the withdrawal of TPA. It is suggested that a prolonged stimulation of the protein kinase C causes a irreversible deregulation of the membrane channel function during cell dedifferentiation.