The acute effects of the partial reduction of coronary blood flow (CBF) on electrical and metabolic changes in myocardium were studied in 59 dogs. In seven dogs with a CBF reduction of 20% to 49%, the adenosine triphosphate (ATP) content (3.59 +/- 0.45 mumol/gm) and the width of the composite electrogram (54 +/- 5 msec) were not significantly different from those of 14 control dogs. In 14 dogs with a CBF reduction of 50% to 74%, ATP decreased significantly (3.09 +/- 0.30 mumol/gm, p less than 0.01); however, widening of the composite electrogram was not noted. Malignant ventricular arrhythmias developed in 5 of 10 dogs with a CBF reduction of 75% to 90% and 6 of 14 dogs with a 100% occlusion, but in none of the dogs with a CBF reduction of less than 75%. Marked widening of the composite electrogram (94 +/- 42 msec, p less than 0.05; and 115 +/- 54 msec, p less than 0.001) and further decrease in ATP (2.49 +/- 0.28 mumol/gm, p less than 0.001; and 2.66 +/- 0.45 mumol/gm, p less than 0.05) were noted in these two groups with a CBF reduction of greater than or equal to 75%. In conclusion, more advanced ischemia was mandatory for electrical derangements than for metabolic deterioration.