Twenty-four caloric vestibular tests were conducted in 20 test subjects during parabolic flight, in which weightless periods of about 10 seconds were elicited. The caloric nystagmus disappeared completely in all experiments in weightlessness, whereas in the higher G-periods the speed of the nystagmus increased. With regard to the modes of response during the periods with increased G-values prior to and following the weightlessness, four different types could be distinguished. In mode I the speed of the slow nystagmus phase increased in the higher G-periods, the nystagmus showed a prolonged duration, while in weightlessness it faded out completely. A reversed nystagmus appeared when the primary caloric nystagmus had disappeared. In modes II, III and IV the direction of the nystagmus reversed during weightlessness. In modes II and III this reversed nystagmus appeared even when the primary caloric nystagmus had disappeared. In mode III the nystagmus resembled the type of mode II, with the difference that it faded away much earlier than could be expected. In mode IV the primary nystagmus lasted rather briefly, while the reversed nystagmus during weightlessness was also much shorter. The experiments produced two remarkable facts: the appearance of a nystagmus reversal, a 'secondary nystagmus', in all experiments in weightlessness, and furthermore great differences in findings between the individuals as well as between experiments in the same person. The only explanation for the inconsistency in the findings is the assumption that effects on other canals, otoliths and vestibular nuclei are continuously changing and therefore interfere with the only endproduct measured, the horizontal nystagmus.(ABSTRACT TRUNCATED AT 250 WORDS)