(1) A small, but statistically significant, increase was observed in the percentage of chromosome aberrations in the rootlet cells of air-dried wheat and pea seeds after germination. In this case only, the increase did not depend on flight duration. (2) Lysogenic bacteria exhibited an increase of genetic alterations and increased phage production. Length of flight was associated with increased bacteriophage production by the lysogenic bacteria. There was an increase of recessive lethals coupled with nonconvergence of chromosomes (sex linked) in the fruit fly. A stimulation of cell division in wheat and pea seeds was observed. Cultures of human cells exposed to space-flight factors did not differ significantly from terrestrial controls with respect to such indicators as proliferation rate, percentage of mortality and morphological, antigenic, and cultural properties. Repeated flights of the identical HeLa cells revealed that there was a longer latent period for restoration of growth capacity than in cells carried into space once or not flown at all. (3) The most definite radiation effects observed were only revealed in genetic tests. No harmful influence on those characteristics affecting the viability of the organism has been discovered. The Air Force Discoverer series launched from the west coast had a few successful flights incorporating organisms. With severe environmental stress and long recovery times, data on radiation exposure were equivocal up to Discoverer XVII and XVIII when cultures of human tissue were flown, recovered, and assessed for radiation exposure effects. Comparison with ground-based controls revealed no measurable differences. Radiation dosimetry from the Mercury series established that minimal exposures were encountered at those orbital altitudes. A typical example is the MA-8 flight of W. M. Schirra, Jr., during which the body surface dosage was less than 30 millirads. NASA has supported fundamental radiation studies at the Oak Ridge National Laboratory and the Lawrence Radiation Laboratory. Emphasis has been placed on the biological effects of high-energy proton radiation and particulate radiation from accelerators. At the NASA Ames Research Center extensive fundamental studies are being carried out on the effects of radiation, especially in the nervous system. It has been demonstrated that deposits accumulate in the brain following exposure to large doses of ionizing particle radiation as well as after X-irradiation. These deposits, referred to as a "chemical lesion," result from an accumulation of glycogen. The formation of these deposits during exposure to large doses of X-irradiation was not increased in environments of 99.5 percent oxygen and increased atmospheric pressure. SIMULATION OF PLANETARY (MARTIAN) ENVIRONMENTS