BACKGROUND: When two shapes are presented successively and briefly to normal subjects, the perceived form of the second (test) shape is changed dissimilar to the form of the first (priming) shape, known as the shape distortion effect. To determine differential effects of aging on visual perception, we measured the perception of briefly presented elemen-tary shapes and of shape distortion in healthy volunteers. METHODS: For each trial, a priming rectangle, a gap of 180 ms, a test circle, and a random-dot mask were flashed in succession. The test circle was flashed in each quadrant. The preceding priming rectangle was flashed at the same position or in the contralateral hemifield. In intermixed trials, there was no priming rectangle and the test shape was a circle or an ellipse. After each trial, the percent elongation [(longer diameter - shorter diameter) / shorter diameter X 100] of the reproduced circle or ellipse was computed. RESULTS: The mean percent elongation in response to control circles and ellipses did not vary with increasing age. The shape distor-tion effect decreased significantly with increasing age during both intra- and contra-hemifield trials. CONCLUSIONS: Decreased shape distortion effect in the presence of normal perception of elementary shapes in the elderly suggests a more severe senescent decline of the higher cortical processing of visual stimuli. Age-related functional deterioration may underlie this differential aging effect on the various levels of visual processing.