The effects of gymnochrome A were tested on the electrical activity of the frog atrial heart muscle. Gymnochrome A (1–5 μM) did not alter the resting potential. Gymnochrome A (5 μM) slowed the initial depolarizing phase of the spontaneously beating action potential. Under voltage-clamp conditions gymnochrome A (5 μM) did not affect the electrical constant of the membrane and the kinetic parameters of the peak Na current (I_Na) recorded in the Ringer solution containing tetraethylammonium (2 mM) and Cd² (1 mM) but shifted the membrane potential at which the current both activated and reached its maximal value toward more negative membrane potentials. It did not alter the reversal potential for I_Na, indicating that the selectivity of the Na channels had not changed. These observations suggest that gymnochrome A binds to the membrane and shifts the activation of I_Na on the voltage axis by modifying the free negative fixed charges present at the membrane surface rather than by occupying a specific site on the Na channel. Photoexcited gymnochrome A transiently triggered an early outward current which lengthened the time-to-peak of I_Na and decreased its amplitude. In addition, photoexcited gymnochrome A blocked the background K current. This is, to our knowledge, the first time that such effects are reported on the cardiac muscle. These observations suggest that the photoexcitation of gymnochrome produces physico-chemical effects which lead to intracellular changes. Further experiments are required to determine their nature.