Negative T waves in the ECG during an acute infection may be due purely to functional variations, making the diagnosis of acute myocarditis difficult. The effects of orthostasis, beta-blockade, and intravenous epinephrine infusion (0.07-0.21 micrograms/kg/min) on T waves and the Q-T interval was studied in 28 young men with acute myocarditis, in 29 persons with functional T-wave abnormalities, and in 30 healthy subjects. Myocarditis-induced negative T waves were never normalized by beta-blockade. Functional T-wave inversions were normalized by beta-blockade in 22 subjects (76 percent). Orthostasis always augmented the T-wave abnormalities that were normalized by beta-blockade. Functional T-wave abnormalities responding to beta-blockade were augmented by epinephrine in 17 subjects (77 percent); but those unresponsive to beta-blockade were normalized in six of seven subjects. The Q-Tc interval was significantly (p less than 0.01) lengthened in subjects with myocarditis (410 +/- 36 ms) compared with controls (384 +/- 27 ms). The beta-blockade test is useful in differentiating between functional T-wave inversions and acute myocarditis. However, functional T-wave abnormalities are not constantly abolished by beta-blockade but may then be normalized by epinephrine. A clearly prolonged Q-Tc interval speaks for a diagnosis of myocarditis.