The properties of the inward current of medulla terminalis-X-organ (MTXO) cells isolated from the Penaeusjaponicus eyestalk were studied with the whole-cell clamp technique in the presence of Ca~(2+) and K~+ channel blockers. The inward currents had a threshold at about-50 mV and peaked at-10 mV. The reversed potential (V_(rev)) was very close to V_(Na), the theoretical Nernst equilibrium potential for Na~+. V_(rev) followed VNa when the external Na~+ concentration was varied and the currents were entirely suppressed by 30 nM tetro-dotoxin (TTX), indicating that it was carried by Na~+. The smooth line of concentration-dependent inhibition of sodium currents by TTX represented the best fit with the Hill equation, yielding an IC_(50) of 2.1±0.1 nM. The values of the half-maximal activation voltage V_h were-20.6±0.5 and -19.3±0.5 mV, respectively, in the absence and presence of 2 nM TTX. TTX had no significant effect on the volt-age dependence of steady-state activation and inactivation of I_(Na). Taken together, the results suggest that the inward current recorded under our experimental conditions was carried by sodium ions flowing through fast voltage-dependent Na~+ channels.