Seventy-six dogs, 43 goats, 211 rats, and 255 guinea pigs were exposed to reflected shock pressures at ambient pressures ranging from 5 to 42 psia in air-driven shock tubes. Animals tolerance, expressed as LD50-one-hour overpressures rose progressively as the ambient pressure was increased. By analysis of the results of this study, combined with those from previous shock-tube investigations, a general equation for the regression this equation and previous estimates of the LD50 pressure for mans tolerance to overpressures of 400-msec duration at an ambient pressure of 12 psia, an equation relating LD50 pressure to ambient pressure was developed for the 70-kg mammal.