We investigated the causal role of executive control functions in the production of brief time intervals by means of a concurrent task paradigm. To isolate the influence of executive functions on timing from motor coordination effects, we dissociated executive load from the number of effectors used in the dual task situation. In 3 experiments, participants produced isochronous intervals ranging from 524 to 2,000 ms with either the left or the right hand. The concurrent task consisted of the production of either a pseudorandom (high cognitive load) or a simple repeated (low cognitive load) spatial sequence of key presses, while also maintaining a regular temporal sequence. This task was performed with either a single hand (unimanual) or with both hands simultaneously (bimanual). Interference in terms of increased timing variability caused by the concurrent task was observed only in the bimanual condition. We verified that motor coordination in bimanual tasks alone could not account for the interference. Timing interference only appeared when (a) more than 1 effector was involved and (b) there were simultaneous task demands that recruited executive functions. Task interference was not seen if only 1 of these 2 conditions was met. Thus, our results suggest that executive functions are not directly involved in motor timing, but can indirectly affect timing performance when they are required to schedule complex motor coordination. (Contains 7 figures.)