The growth and geotropic movements of roots and hypocotyls of lettuce have been studied on board the Salyut 7 station in a stationary position and on the centrifuge at 0.01, 0.1 and 1 g. On the centrifuge at 0.1 and 0.01 g as well as under weightlessness, the final length of hypocotyls was by 8-16% greater than in control plants on the centrifuge at 1 g. The length of roots, however, was reduced by 17% at 0.01 g and under weightlessness; at 0.1 g their growth is much the same as at 1 g. On the Earth, while growing in a vertical position, and in space at 0 < or = g, the roots and hypocotyls deviate from the longitudinal axis of the seed. Average values of deviation eagles on the Earth are always equal to zero, while this is not always the case in space, which indicates the biological effect of microgravity conditions on board a spacecraft. The threshold of geotropic sensitiveness of lettuce hypocotyls, calculated from the linear regression parameters of the dependence of the response geotropic reaction upon the value of the centrifugal force, comprised 2.9 x 10(-3) g. In the Fiton 3 micro-greenhouse under spaceflight conditions, the plants of Arabidopsis thaliana (L) Heynh have, for the first time, undergone a full cycle of individual development. The seeds sown during the flight germinated, performed growth processes, formed vegetative and generative organs and, judging by the final result, they succeeded in fecundation, embryogenesis and ripening. Despite the noted modification of growth and development of plants in space, 42% of formed seeds appeared to be valuable biologically.