A total of 455 mlce, rats, gulnea plgs, and rabbits was subjected to lmpact at velocitles ranglng between 25 ft/sec and 51 ft/sec. The deslred velocities were generated by allowlng the anlmals to free-fall from various helghts to a flat concrete pad. The ventral surface of each anlmal was the area of impact. Problt analyses of the 24-hr mortallty data ylelded LD/sub 50/ lmpact veloclties with 95% confldence limits as follows: mouse, 39.4 (37.4 to 42.0) ft/ sec; rat, ft/sec; and rabbit, 31.7 (30.2 to 33.3) ft/sec. The LD/sub 50/ figures for the mouse and rat were slgnificantly higher, statlstically, than those for the gulnea plg and rabblt. The small spread ln the LD/sub 50/ values suggested little variation ln the tolerance of blologlcal systems to impact. Further, the steepness of the mortallty curves lndicated a narrow survlval range to lmpact. Extrapolatlon of the experimental data to the 70 kg anlmal yielded a predicted LD/ sub 50/ impact velocity of 26 ft/sec (18 mph). Literature relevant to the human case was revlewed and the tentative appllcabllity of the predlcted flgures to adult man ls discussed. (auth)