Measurements of the threshold and growth function of the acoustic reflex in man were made utilizing a signal-averaging technique. Pure tones (250, 500, 1000, 2000, and 4000 Hz) and broadband noise served as the contralateral stimuli delivered at intensity levels from the reflex threshold to 116 dB SPL. The acoustic conductance and susceptance components of admittance, at 220 and 660 Hz, were used to measure the reflex activity. The results are reported as changes in conductance, susceptance, and admittance in cgs mmho. The acoustic reflex thresholds were similar to those reported in other studies, with the 220-Hz probe yielding thresholds that averaged 3.5 dB higher than those obtained with the 660-Hz probe. Broadband noise and 1000-Hz stimuli produced the largest reflex magnitudes, while the smallest were observed with 250- and 4000-Hz signals. The dynamic ranges of the restricted growth functions were frequency dependent for pure tones and ranged from greater than 16 dB with 250 Hz to greater than 28 dB with 1000 Hz, while that for noise was greater than 50 dB. The first derivative of the best-fit third-degree polynomial was used to describe the slope characteristics of the growth functions, in which frequency and intensity effects were found. Relationships between the acoustic reflex and tympanometry were observed and are also discussed.