Four experiments in the aft compartment of Biosatellite II investigated the broad question of the effect of nearly zero gravity on the development, morphology and metabolism of plants and animals. The fertilization and development of the egg of a vertebrate (the frog, Rana pipiens), the feeding and growth of a protozoan (the giant amoeba, Pelomyxa carolinensis), the orientation of leaves and petioles of a young dicotyledon (pepper plants, Capsicum annuum) and the morphogenesis, orientation, histochemistry and biochemistry of a monocotyledon seedling (wheat, Triticum vulgare) gave a broad scope. All are known to have specific responses to normal gravity and changes in them might be expected to reflect the effects of orbital flight on living organisms. No differences in development of the frog eggs could be detected. Unfortunately, the 3 1/2 hour delay in launch allowed the first cleavage (the stage most sensitive to inversion) to appear before launch. Although the orbited embryos were somewhat slower to reach certain stages of development, recovered embryos developed just as did the controls. The amoebae fed normally while in orbit, and specimens fixed in orbit retained the ordinary heteropodal shape. Growth rates of orbited amoebae, both fed and starved, were slower than controls following reentry and recovery procedures. In continuous-fed organisms there was little or no effect of flight detectable in growth rate or actual number of divisions. Electron micrographs showed no abnormalities and few differences between flight and control organisms. The pepper plants were photographed in orbit at ten-minute intervals, as were the clinostat and erect controls. The subsequent measurement of photographs showed that in the orbited plants all leaves showed epinasty, the interaxial angle decreasing by 20-60 degrees C. Plants on the horizontal clinostat behaved comparably, but recovered more rapidly than orbited plants when returned to the normal erect position. Although the maximum age of wheat seedlings was only 65 hours, coleoptile and root growth rates during that time had not been significantly altered by flight or by slow rotation on a horizontal clinostat. There was some evidence that growth was accelerated after normal gravity was restored. The orientation of coleoptiles and of primary and lateral roots of orbited plants varied significantly from the normal erect seedlings but was almost identical with that of clinostat plants. The Periodic-Acid-Schiff technique on sectioned material showed starch grains at the bottom of cells of erect control coleoptile and root tips, while in orbited and clinostated plants the grains were located more or less at random. Histochemical differences between clinostat and orbited tissues are apparent however. Peroxidase localization varied and its activity was higher in both clinostat and orbited tissues; five other enzymes studied biochemically showed no differences. These experiments all suggest that there is no deleterious effect on living organisms or their activities from short-term weightlessness. Several results indicate that the horizontal clinostat may simulate the weightless state effectively here on Earth.