In tadpoles of Xenopus laevis, the effects of microgravity on the development of the roll-induced vestibuloocular reflex (rVOR) was investigated. Special attention was focused on sensitive periods and the minimum duration of microgravity exposure by which the development of the rVOR is affected. The peak-to-peak excursion (rVOR amplitude) of the rVOR characteristic for a lateral 360 degrees roll was used to describe microgravity effects. Fertilization of all eggs was performed 40 h before launch. Tadpoles were exposed to microgravity either during the first (MC-group) or second half of the mission (CM-group), or throughout the 9-day mission (MM-group). Inflight, 1G-gravity was simulated by a centrifuge (CC-group). After termination of the mission, the rVOR amplitude was only reduced in the MM-group with respect to the 1 G-inflight and 1 G-ground control by approximately 20-30% while both the MC- and CM-groups were not affected by the 4-day and 5-day microG exposure, respectively. However, CM-tadpoles like MM-tadpoles showed malformation of their body characterized by a dorsal bended tail. It disappeared in both groups within 2 weeks after landing. The difference between the rVOR amplitudes of the experimental groups disappeared within 5 weeks after landing. The results demonstrate that microgravity retards the development of the rVOR if it lasted longer than 4 days but that tadpoles are susceptible even for shorter periods as shown by the malformation of the body.