A four alternative Choice Reaction Task, oil alternatives equally likely, was used to study the effect of size and contrast ratio of the visual stimulus on response time. A full factorial experiment was designed in which three levels of both the variables were tested. Sizes subtending visual angle of 1 min, 3 min, and 5 mill and the contrast ratios of 0-94, 0-75, and 0-34 were used. Seventeen subjects wore tested for this experiment and each subject responded 250 times under each experimental condition. The analysis has indicated that the contrast ratios tested did not significantly (a = 0'05) effect the response time.