Two right-handed and 2 left-handed participants drew circles in the horizontal plane with both hands simultaneously in either a symmetrical or an asymmetrical mode, at their preferred rate or as fast as possible. During symmetrical movements, the hands showed frequency and phase synchronization at both rates. During fast asymmetrical movements, the hands showed increased phase difference and phase variability, as well as transitions to symmetrical movements, and cases of frequency decoupling. Large distortions of the hand trajectory were also observed under fast asymmetrical movements. Trajectory distortions and movement direction reversals were confined to the nondominant hand. Under the assumption that circular trajectories are generated by properly timed orthogonal oscillations along the y-axis and the *-axis, these findings are accounted for by the characteristics of coupling between homologous functional oscillators of the 2 body sides. Much recent research on motor control has been devoted to identifying the constraints imposed on interlimb coordination when the limbs have to be moved simultaneously. Those that have been by far the most extensively studied are the temporal constraints involved in bimanual activities. Studies on this topic have shown, for example, that bimanual tapping is easy when the two hands produce identical or harmonically related rhythms (e.g., 1:1, 2:1, and 3:1) but extremely difficult when the two rhythms are not integer multiples of each other (e.g., Klapp, 1979; Peters, 1977; Summers, 1990). The studies just cited dealt with cases in which similar movements were performed by the two hands and only the timing structure was varied. In other studies, the task has consisted of performing different movements with both limbs but in the same overall movement time. For example, in a series of experiments, Swinnen and colleagues asked their participants to perform a horizontal elbow flexion (unidirectional) movement with the nondominant limb at the same time as an elbow flexion-extension-flexion (reversal) movement with the dominant limb (e.g., Swinnen, Walter, Beirinckx, & Meugens, 1991; Swinnen, Walter, &