Na(+) currents were measured in myocytes from a fetus with congenital myotonic dystrophy type 1 (DM1) using the patch-clamp whole-cell technique. Steady-state activation and inactivation properties of Na(+) channels were not substantially different between these cells and age-matched control cells. However, a decrease in Na(+) channel density and a faster rate of recovery from inactivation were found in myocytes from congenital DM1 suggesting that changes in functional Na(+) channels may affect cell excitability of muscle cells of patients with this disorder.