Seventy-two healthy young individuals were subjected to controlled, moderate hyperventilation with room air and with 4.9 percent CO2 in air, and monitored electrocardiographically. Significant summed frontal T-wave changes with hyperventilation (sigmaT1,2,3 larger than or equal to 1.5 mm) were observed in 12 patients. Six subjects (8.3 percent) showed T-wave depression. It was reversed in five patients by hyperventilation with 4.9 percent CO2 in air. T-wave elevation, observed in six subjects, was reversed in four patients by hyperventilation with 4.9 percent CO2. A short period of hyperventilation with an air mixture containing 4-5 percent CO2 is suggested as a means of screening patients under suspicion of ischemic heart disease exclusively on the basis of ECG changes.