Motion sickness susceptibility of five labyrinthine-defective (L-D) and 25 normal subjects was tested under the force environments encountered in parabolic flight (0 g and hyper-g). The L-D subjects were uniformly symptomless, while the normal subjects revealed great inter- and intra-individual differences in susceptibility to motion sickness provoked by standardized head movements during: (1) the hypergravic and (2) the weightless phases of the parabolic maneuver while restrained; and (3) the weightless phase while being rotated in a chair. Four of six subjects tested under condition 1 were completely unaffected by the condition while two reacted with symptoms. Condition 2 provoked severe symptoms in five of the twelve subjects tested and moderate symptoms in one. Fifteen subjects tested under condition 3 revealed either a marked increase or decrease in susceptibility to Coriolis acceleration in weightlessness compared to terrestrial baseline measurements. (Author).