1. The effects of Droperidol on ionic currents were studied in frog atrial fibres by means of double sucrose-gap technique associated with an optical method for recording contraction. 2. Droperidol (5-10(-5) M) caused a significant reduction in the amplitude of the inward currents and slowed their inactivation kinetics. The decrease in the sodium and calcium current intensity may be related to the reduction in the maximum conductance for these ions. 3. A decrease in the amplitude of the first component of contractile response was shown to depend upon a reduction of the intensity of the calcium current. 4. Droperidol affected neither the potassium outward current nor the second component of the contractile response. 5. Repetitive activity induced by long-lasting depolarizing current pulses was suppressed by Droperidol. This suppression may be related to the decrease in the maximum conductance for sodium and calcium ions. 6. Droperidol increased the delay of the reactivation of the inward currents and prolonged the refractory period. 7. These results suggest that the cardiac properties of Droperidol are mainly due to a decreased conductance to Na and Ca ions.