The effect of immersion stress on guanidino compounds in rat organs was studied. Short-term (5min) immersion stress increased the creatinine level in the cerebral cortex and the homoarginine level in the cerebellum, but it did not affect guanidino compound levels in the liver, kidney and pancreas. On the other hand, long-term (60min) immersion stress decreased the creatinine level in the liver and the arginine level in the kidney, and increased the arginine and γ-guanidinobutyric acid levels in the liver. However, long-term immersion stress did not affect guanidino compound levels in the brain or pancreas. Further, long-term immersion stress increased the excretion of γ-guanidinobutyric acid in the urine. These findings suggest that the transient increases of creatinine in the cerebral cortex and homoarginine in the cerebellum might be related to a functional change in the central nervous system during the early stage of immersion stress. It is thought that the long-term immersion stress affects guanidino compound metabolism in the peripheral organs through changes in protein metabolism.Administration of imipramine reduced the lowered mood induced in rats by immersion stress, but it did not affect the change in guanidino compound levels induced by long-term immersion stress. This result suggests that the alleviation of the lowered mood induced in rats by imipramine treatment is not related to changes in guanidino compound levels in rat organs.