By using two-microelectrode voltage clamp technique, the effect of noradrenaline and acetylcholine on transient inward current (ITi) induced by acetylstrophanthidin was observed in sheep cardiac Purkinje fibers. In the presence of acetyl strophanthidin (4.5 X 10(-8) mol/L), the stability of ITi could be maintained for about 1.5 h. Noradrenaline (1.5 X 10(-6) mol/L) increased the ITi from 11.4 +/- 2.5 nA to 14.5 +/- 4.1 nA (n = 11, P less than 0.01) and shortened the ITi duration from 688 +/- 278 ms to 556 +/- 237 ms (n = 11, P less than 0.05). Acetylcholine (2.5 X 10(-6) mol/L) decreased the ITi amplitude from 13.4 +/- 3.8 nA to 10.6 +/- 3.2 nA (n = 8, P less than 0.01) and prolonged the ITi duration from 653 +/- 193 ms to 741 +/- 222 ms (n = 8, P less than 0.05). Acetylcholine (2.5 X 10(-6) mol/L) also decreased the ITi amplitude in the preparation pretreated with noradrenaline (2 X 10(-6) mol/L) from 14.8 +/- 7.7 nA to 10.0 +/- 6.7 nA (n = 6, P less than 0.01). In addition, the mechanism of noradrenaline effect was also studied. When the slow inward current (Isi) was blocked by Mn2+ (0.75 mmol/L), noradrenaline still increased the ITi amplitude from 2.2 +/- 0.2 nA to 4.9 +/- 1.1 nA (n = 6, P less than 0.01) but had no effect on Isi, suggesting that noradrenaline has a direct effect on ITi.