We compared horizontal and vertical smooth pursuit recorded with a scleral contact lens technique at multiple sinusoidal frequencies (0.2-1.6 Hz) and peak velocities (12.5-100 degrees.s-1) in 10 normal subjects. The gain was significantly higher (p less than 0.05) and the harmonic distortion significantly lower (p less than 0.05) for horizontal pursuit compared to vertical pursuit for all stimuli except the lowest frequency and peak velocity. Both horizontal and vertical pursuit exhibited a phase lag that was minimal at lower frequencies but reached a mean of about 37 degrees at 1.6 Hz. This difference between horizontal and vertical pursuit could be explained by current models of the smooth pursuit system if one postulates a lower saturation level of retinal error velocity for vertical pursuit compared to horizontal pursuit.