The purpose of this study is to investigate the changes of Leu-Enkephalin (L-Enk) content in striatum and hypothalamus of rats during hyperbaric (HBO) oxygen exposure. Thirty-two male rats in the experiment were randomly divided into four groups: normobaric air group, normoxic hyperbaric nitrox group, nonconvulsion HBO group and convulsion HBO group. L-Enk content in striatum and hypothalamus was determined by radioimmunoassay. The results show that L-Enk content in the striatum and hypothalamus of rats exposed to hyperbaric oxygen environment were markedly higher than that of rats exposed to normobaric air and normoxic hyperbaric nitrox. The experimental results suggest that the elevation of L-Enk content in striatum and hypothalamus shows a positive relationship with the HBO exposure duration of animals and that there was no marked relationship with normoxic hyperbaric nitrox environment and compression-decompression method.