The development of susceptibility to immobilization-induced gastric erosions was studied in laboratory rats previously separated from their mothers at 15, 21 or 25 days of age. Early separation (day 15) produced animals whose maximum susceptibility occurred at a much younger age, generated a susceptibility curve over life that was the inverse of the curve for animals separated later and led to severe gastrointestinal hemorrhage as a common and distinguishing complication in younger rats. The pathogenesis of erosion formation in early separated rats may be unique in that, for that group only, the food deprivation component of the immobilization paradigm, when presented alone, also produced erosion of the glandular stomach, with hemorrhage.