Concussion resulting from head acceleration could explain the poor survival rates in some types of accidents. Experiments have been conducted on a decelerator using a tracking task to determine whether high head acceleration could affect psychomotor performance. Human subjects were exposed to impact acceleration of O (sham), 5, 10 and 12 -Gx facing forwards. Measurements were made of the linear and angular accelerations experienced at the head and a step tracking task was used to examine psychomotor performance. Electroencephalographs were also recorded. Both the linear and angular accelerations at the head were increased at the higher levels of impact acceleration. At -5Gx there were no significant differences in psychomotor performance when compared with controls, but at -10Gx, and especially -12Gx, significant differences were found. The EEG activity did not vary significantly and no concussive effects were observed in any subject. These results suggest that impairment of psychomotor performance severe enough to jeopardise survival could be produced by high accelerations of the head, though neither linear nor angular acceleration appear to have special significance.