The modification of the development of the high pressure nervous syndrome (HPNS) has been demonstrated by varying the external environment (chamber temperature and compression rate) or the internal environment (core temperature, pharmacology and age). The effects of compression rate on the convulsion-threshold pressure and EEG activity in 68 adult male Wistar rats with chronically implanted electrodes were studied. Restrained animals were individually compressed at a predetermined rate in He-O2 to a simulated depth of 4500 fsw [feet of sea water] (137 ATA [atmospheres absolute pressure]), with colonic temperature maintained at normal levels. Six compression rates showed that convulsion-threshold pressure for euthermic rats did not significantly differ between any of the groups.