The present work studied the effect of veratrine on electrical stability of the mammalian ventricles. In a group of anesthetized open-chest dogs we determined the effects of veratrine (0.001-0.3 mg/kg) on the threshold for ventricular fibrillation (VFT), both during sinus rhythm (SR) and during atrial drive (AD) at 166 +/- 7 beats/min. Veratrine (0.3 mg/kg) increased the VFT during SR from control levels of 33 +/- 5 mA to 56 +/- 4 mA (P less than .05) and during AD from 34 +/- 6 to 57 +/- 9 mA (P less than .05). VFTs after infusion of veratrine were higher than in a control group of dogs given the saline diluent in which VFT during SR was 26 +/- 3 mA (P less than .001) and during AD was 24 +/- 3 mA (P less than .005). The increase in VFT during SR was independent of the decrease in heart rate, which fell from 147 +/- 8 to 106 +/- 6 beats/min (P less than .001). It was associated, however, with an increase in the QT interval during SR from control values of 254 +/- 13 to 308 +/- 19 msec (P less than .02) and during AD from 238 +/- 8 to 267 +/- 8 msec (P less than .005). We conclude that small doses of veratrine increase the electrical stability of the ventricles in the normal dog heart.