Three groups of men were tested in a situation involving reaction to two successive stimuli at a short known time interval. Each reaction required the choice of one of two reaction keys, depending upon the light stimulus delivered. For one group of subjects, the first reaction was made easy by making the left key correct for the left light; for the second group, the first reaction was made slower by making the left key correct for the right light. In the third group, one of the two possible stimuli for the first reaction occurred more often than the other, so that the first reaction was sometimes fast and sometimes slow. The second reaction was delayed if the interval between stimuli was shorter than the first reaction time; in conditions where the first reaction time was longer, the second reaction time was delayed by an even greater amount. This result supports the view that a common decision-making mechanism of limited capacity is occupied by the first reaction and so is unable to deal with the second one; two other theories of the effect seem inconsistent with the present data.