This study was undertaken in order to examine the effects of inter-stimulus interval and accuracy demands of primary task on the psychological refractory period. Subjects consisted of six male right-handed voulunteers with a mean age of 25 years graduate students who performed on a double stimulation task . A 3×2(inter-stimulus interval×target size) within subjects design was used for this study. As predicted, as the ISI was increased RT₂ progressively shortened, and the delay in RT₂increased as the target size reduced. The noted decrease in magnitude of RT₂ delay as ISI(inter-stimulus interval) increased would be consistent with prediction of single channel theory. Further, the finding that RT for the small target movement was larger than that for the large target movement suggest that the small target required increased attention, perhaps because of the additional planning and control necessary to hit it. Thus a slower RT₂ indicated increased capacity being devouted to the primary task. The results also showed that the interval between the two responses decreased as the time between stimuli decreased. The implications of these findigs are discussed in terms of single channel hypothesis.