Subjects used smooth eye movements to track small-amplitude sinusoidal target motions. Target frequencies (0.05 to 5 Hz) and amplitudes (1.9 to 30 min of arc) were in the range of those found in the retinal image during fixation of a stationary target while the head is not artificially supported. Smooth pursuit was poor at high target frequencies in several ways: Large uncompensated drifts were observed for target frequencies between 1 and 4 Hz. The drifts were superimposed upon oscillations of the eye in response to the target motion. Mean retinal-image speeds were higher than retinal-image speeds during slow control (smooth eye movements with stationary targets) for target frequencies above 0.5 Hz. Mean retinal-image speeds were as high as target speed for target frequencies above 3 Hz. The ratio of eye speed to target speed decreased as target frequency and amplitude increased. The dependence on amplitude could be reduced and often eliminated by computing an adjusted ratio in which a constant (approximately equal to the mean speed of slow control) was subtracted from eye speed before dividing by target speed. Adjusted ratios declined for frequencies above 0.5 to 1 Hz and did not depend on amplitude. These results show that the response of the smooth-pursuit subsystem to target motion above 0.5 Hz is poor, even though the velocity and the acceleration of th motions are low. Models of smooth pursuit in which the response of the eye depends exclusively on the velocity, acceleration, or position of the target do not account for our results.(ABSTRACT TRUNCATED AT 250 WORDS)