We have analysed the voltage gated ion channels and fusion competence of GFP/PKH labelled skeletal muscle myoblasts transplanted into the infarcted myocardium of syngenic rats. After cell transplantation the animals were sacrificed and GFP(+)/PKH(+)-myoblasts enzymatically isolated for subsequent studies of ionic currents through voltage gated sodium, calcium and potassium channels. A down regulation of all three types of ion channels after engraftment was observed. The fraction of cells with calcium (68%) and sodium channels (65%) declined to zero within 24 hours and 1 week respectively. Down-regulation of potassium currents (90% in control) occurred within 2 weeks to about 30%. Before injection myoblasts expressed predominantly transient outward potassium channels whereas after isolation from the myocardium exclusively rapid delayed rectifier channels. The currents recovered completely between 1 and 6 weeks under cell culture conditions. The down-regulation of ion channels and changes in potassium current kinetics suggest that the environment provided by infarcted myocardium affects expression of voltage-gated ion channels of skeletal myoblasts.