OBJECTIVE: To determine whether the impairment of perceptual-motor performance often observed during the initial stages of space flight is attributable to the direct effects of microgravity on sensory-motor input-output relationships or to cognitive overload arising from the variety of stressors encountered in this environment. BACKGROUND: Experimental evidence is contradictory, and the present experiment investigated the role of two variables: use of ajoystick with tracking or aiming tasks and degree of arm restraint. METHODS: Five well-trained astronauts performed Fitts' reciprocal aiming task on three occasions (preflight, on the International Space Station, and postflight) under the following conditions: (a) with a stylus or ajoystick, (b) with the arm restrained or unrestrained, and (c) as a single task or together with auditory reaction time (RT) as a dual task. In addition to the speed and accuracy of responses, kinematic measures were derived from the joystick. RESULTS: In space, the slope of Fitts' function increased only in the dual task condition with the joystick. RT was also slowed in the dual condition, and there was an increase in aiming and RT errors. Percentage time to peak acceleration, velocity, and deceleration were increased when the aiming task was performed alone. CONCLUSION: These results support the cognitive overload rather than the microgravity hypothesis and indicate that an impairment in sensory-motor performance is not a necessary concomitant of space flight. APPLICATION: Impairments in perceptual-motor performance in space can be eliminated or mitigated by appropriate training and task design.