Repeated measurements of reaction times were made individually in 53 healthy subjects at 60-sec intervals and continued for 30 min. The subject’s visual perceptual task each minute was to determine the momentary cessation of the second hand of a clock and his reaction times were automatically recorded. The results showed that the subject’s reaction times followed a free-running cycle ranging in frequency between 4 and 15 min when their reaction times were submitted to spectral analysis and autocorrelograms obtained. Error scores during the procedure showed significant correlations with chronological age but not with gender. The biological implications of these oscillations are discus