An automatic gain control (AGC) for maintaining constant gain of the i.f. amplifiers in an ultrasonic pulse‐echo system is described. The AGC stabilizes the receiver against long‐term drifts. It is applicable if the duty cycle of the sonic‐echo train is small (5% or less) and the rest of the cycle is available for introduction of a reference signal. The AGC samples the amplified reference signal and keeps its level constant, thereby maintaining constant gain in the i.f. amplifiers without the necessity for using an additional gated integrating amplifier. The AGC improved the reliability of measurements of sonic‐echo voltage from ±3% without the AGC to ±0.5% with the AGC over a long period of time. The total ultrasonic system for attenuation measurements in the range 20–500 MHz, including the AGC, is described.