Effects of body position on auditory threshold acuity, the acoustic impedance at the tympanic membrane, and the middle ear muscle reflexes were investigated at 150, 250, and 500 Hz. Relative to the values obtained in the seated upright position, threshold acuity was reduced, the resistive and reactive components of the acoustic impedance were greater, and the effects of stapedius and tensor tympani muscle contractions on the compliance at the tympanic membrane were reduced in the inverted (upside-down) position. The increase in acoustic impedance, which is probably due to an increase in the hydrostatic pressure of the labyrinthine fluids, accounted for only about half the decrease in threshold acuity.