To investigate the character and extent of the errors due to poor frequency response that present-day equipment may introduce in the amplification of the electrocardiographic signal, a single, electronically simulated electrocardiogram complex was recorded by means of an amplifier with a variable frequency response. Poor high-frequency response resulted in attenuation of the R and S waves, and the attenuation increased in a smooth curve with decreasing frequency response. However, the R and S were attenuated by different factors for any given value of high-frequency cutoff. Inadequate low-frequency response gave rise to R, S, and I amplitude distortions that again followed a smooth curve with degenerating frequency response, and these waves were distorted by different factors for any single value of low-frequency cutoff. In addition, poor low-frequency response inserted S-T segment depressions and post-T sags. Much equipment in current use is capable of producing such distortions to a significant extent.