When two stimuli are presented in rapid succession and a subject has either to react to both or to the second stimulus only, the second reaction time increases with decreasing interstimulus interval. Several theories have been put forward to explain this phenomenon. Experiments are described with which the expectancy theory and the intermittency theory can be tested. From the experimental data it is argued that the effect of expectancy is to be neglected with interstimulus intervals shorter than .6 of a second. There appeared to be a clear discrepancy between the predicted data based on an internittency hypothesis and the experimental data.