The effects of helium-oxygen (He-O2) environments were observed upon the pharmacological actions of morphine in male albino rats given 100 mg/kg of body weight. Measurements of 8-h urine volumes revealed that animals maintained in ambient air and He-O2 at 1 ATA exhibited a marked antidiuresis. Animals exposed to He-O2 at 11 ATA showed a normal urine flow when compared with ambient air controls. Uninjected rats maintained in He-O2 at 11 ATA showed a marked diuresis. The study also included experiments to determine whether the excretion of free morphine is altered in He-O2 environments, and whether the analgesic effect of the drug is changed due to He-O2 environments. Exposure to He-O2 at 21 ATA was included in these studies. Significant decreases were noted in the excretion of free morphine during a 24-h period in those animals exposed to 11 and 21 ATA, although no difference was observed in the groups maintained at 1 ATA. Pressure was also shown to decrease the pain threshold in both uninjected and injected animals maintained at 11 and 21 ATA. The results indicate that the metabolism of morphine may be stimulated due to the effects of pressure.