The parietal mechanisms for on-line control of hand trajectory were studied by combining single-cell recording and reversible inactivation of superior parietal area 5 (PE/PEc) of monkeys while these made reaches and saccades to visual targets, when the target moved in space. Neural activity in these areas was related to hand and/or eye movement and position. The activity of reach-cells mostly led the onset of hand movement and was related to position, speed and movement direction. The activity of eye-related cells was both pre and post-saccadic. Under bilateral muscimol injection an increase of the hand reaction- and movement-time to the first and second target occurred, together with a temporal shift of the hand velocity profile. This caused an increase of the time necessary for trajectory correction, therefore an elongation of the hand-path toward the first target location. Furthermore, hand trajectories were different in shape and more spatially dispersed than control ones. In contrast with the hand, the eye moved to the first target and then to the second one without adjusting its trajectory. However, the saccades to the first and second target were both delayed in time. These results identify the Superior Parietal Lobule as a crucial node in the on-line control of hand and eye movement , especially when they are both set in motion for reaching to visual target, and provide an experimental model of Optic Ataxia in monkeys.