We sought to measure separately the motor potentials for each of 2 concurrent tasks and to use these measurements to identify the locus of dual-task interference. Lateralized readiness potentials (LRPs) were measured in the psychological refractory period paradigm, in which a separate response is required to each of 2 successive signals. As the interval between the signals decreased, the 2 reaction time (RT) tasks increasingly overlapped and the 2nd RT was prolonged. The LRP for the 2nd task was also delayed but maintained a constant temporal relation with the 2nd RT and sometimes preceded the 1st-task RT. The results indicate that (a) independent measures of the LRP can be obtained for each of 2 concurrent tasks, (b) slowing of the 2nd task was caused by a delay in processes that precede LRP onset, and (c) the 1st task may cease to interfere with the 2nd considerably before producing an overt response.