The influence of the varying gravito-inertial (Gz) force during parabolic flight on human otolith function was investigated experimentally. It was hypothesised that a varying Gz force profile initiates an otolith-ocular response that manifests itself in modulation of optokinetic nystagmus slow-phase eye velocity (OKN-SPV). Six subjects were seated in the ESA-Caravelle, facing perpendicular to the aircraft's longitudinal axis. The Gz profile was subsequently 1.8 Gz pull-up, 0 Gz microgravity, and 1.8 Gz recovery, each phase lasting about 20 s. Vertical eye movements were recorded with electro-nystagmography throughout the parabolic manoeuvre. Conditions were: (1) visual fixation, (2) darkness and (3) optokinetic stimulation of 50 deg/s in an upward or downward direction, projected on a cylindrical screen at 0.6 m viewing distance. No consistent nystagmus or gaze shift was measured in darkness. With optokinetic stimulation, however, ANOVA revealed downward enhancement of OKNSPV by 5 degrees/s in 1.8 Gz hypergravity, as compared with the 0 Gz condition and the 1 Gz condition. It is concluded that an otolith-ocular pathway modulates optokinetic eye movements in parabolic flight.