Marked body size reduction (minus 50.5%), and delayed head and body skeletal development were noted in albino rat embryos following exposure of the mother to +/- 3 G acceleration for 3 hr/day between the 10th and 19th day of pregnancy. In addition, electron microscope examination of the inner ear showed signs of greater functional activity of the labyrinth, unaccompanied by reduction of development. Semicircular canal cells and size, in fact, were normal in the crests and at a distance from the latter. There was a marked increase in the number of mitochondria in the cells, in ER area and volume, and A and B glycogen granule richness compared with the controls, these being all signs of considerable activity. The findings substantiate prior histological and histochemical observations: the physiological stimulus offered by acceleration is highly specific for the inner ear structure, particularly that of the labyrinth, to the point where the handicap imposed by the circulatory hypoxia caused by such acceleration, by disturbing the mother-foetus circulation, is overcome.