Ninety-six Ss were tested on a lateral-tilt chair under darkroom conditions. The task consisted of 24 trials and required 5 to return himself from a position of 30° tilt to the perceived vertical. It was found that: (a) Ss with repeated testing demonstrated a reduction of error in perception of the vertical, probably as a result of proprioceptive learning; (b) return to the true vertical following each trial judgment (postural knowledge of results) and visual representation of error (visual knowledge of results) both had a significant positive effect upon reduction of error; (c) Ss tilted alternately left and right performed significantly superior to 5s tilted repeatedly to the left. In the case of the alternation 5s it was further found that neither visual nor proprioceptive knowledge of results were contributing anything additionally to reduction of perceptual error than that provided by alternation alone. An attempt was made to explain this finding in terms of experiential factors and competition between sensory events resulting, in the case of the alternation group, from the mere process of passing through the true vertical. Finally, implications of the findings for the design of spatial orientation studies were discussed. (PsycINFO Database Record (c) 2006 APA, all rights reserved)