The control of horizontal head and eye movements was examined in 13 nondemented patients with Parkinson's disease (PD) of mild to moderate severity. During pursuit of single-frequency sine waves, smooth component eye velocity was lower in the PD group at frequencies of 1.2 Hz and above; but the differences in overall eye displacement were even greater, indicating an impaired ability to generate catch-up saccades at high frequencies. A corresponding deficit in saccadic performance was observed during a high- frequency saccadic tracking task where predictive saccades of reduced gain and variable timing were generated. During pursuit of pseudo-random target motion with varying degrees of predictability, small differences in smooth component eye velocity were observed, but prediction was otherwise well preserved in the patient group. Vestibulo-ocular reflex (VOR) suppression was also normal during head-free pursuit. No major improvement in smooth pursuit gain could be attributed to drug treatment, based on a comparison of patient results before and after administration of levodopa.