This study investigated changes in the sway of various body landmarks and the activity of various postural muscles during periodic floor oscillation. Subjects (15 healthy young adults) maintained a standing posture with eyes closed on a force platform that oscillated in the anteroposterior direction at 0.5 Hz with 2.5-cm amplitude for 60 s (one trial). Trials were repeated until postural control had adapted (range, 5-14 trials). Sway of various body landmarks, fluctuation of the center of foot pressure in the anteroposterior direction (CoPap) and electromyography of postural muscles were measured. The delay between the anteroposterior sway of body landmarks and the anteroposterior movement of the floor was largest for higher landmarks, regardless of the trial number (tragus, 238.6 ms; lateral malleolus, 2.3 ms). For all landmarks expect the lateral malleolus, the amplitude of the frequency spectrum of landmark sway at 0.5 Hz significantly decreased with adaptation, and the decrease was larger for the higher landmarks. Cross-correlation coefficients of landmark sway and floor movement were smaller for higher landmarks regardless of the trial number (mean cross-correlation coefficient of first and final trials: tragus 0.667, lateral malleolus, 0.996), with the larger amplitude of the frequency spectrum at about 0.1 Hz. The peak activity of all muscles significantly decreased with adaptation, and the decrease was largest in the tibialis anterior (first trial, 8.0% maximum; final trial, 2.7% maximum) and the gastrocnemius (first trial, 38.6% maximum; final trial, 18.7% maximum).