The influence of the viscosity of a manipulandum used by a human operator in a position-control pursuit-tracking task was examined. An active servo-system was used to set the viscosity of a manipulandum (motor) connected to the forearm to one of seven levels ranging in a geometric series from 12 to 800 N.s/m. During each condition the viscosity of the motor was held constant by a computer while subjects tracked, by moving their forearm in the sagittal plane, a visually presented target whose position changed randomly every 1.5 s for 255 s. Nonparametric and parametric impulse response functions were calculated between the input (target) and output (position) in each tracking condition. Nonparametric analyses revealed that subjects became sluggish at higher viscosities (above 200 N.s/m) and took longer to reach the target. A second-order low-pass transfer function was found to provide a very good description of tracking performance at each viscous level. The gain and damping parameter of this transfer function were not affected by the manipulandum's viscosity, whereas both the pure delay and natural frequency of the human operator system decreased systematically with increasing manipulandum viscosity. These findings suggest that over the range of viscosities studied, there is no speed-accuracy trade-off in terms of determining an optimal level of manipulandum viscosity for a human operator, and that a less viscous interface will result in faster performance.