Catheterization of the femoral artery using rubber band restraint markedly lowered growth hormone (GH) levels in plasma of male rats which had been anesthetized with pentobarbital (PB). Other stressful factors existing in the process of catheterization, such as skin incision, soft tissue separation to expose the femoral artery, restraint on the back, catheterization with scotch tape restraint and catheterization without restraint, did not produce a significant suppression of plasma GH levels. On the other hand, in all catheterization a marked increase in plasma corticosterone (CS) levels was observed. When the tibia was broken in anesthetized rats, plasma GH levels dropped within 15 min, then gradually increased. However, use of a rubber band tourniquet for a long period caused a marked suppression of GH within 15 min which was maintained for 75 min. If the rubber band was removed 15 min after application, plasma GH levels gradually increased to pre-stress levels in 60 min. A spinal cord section prevented the suppressive effects of the rubber band tourniquet and tibia break on GH release. These results indicate that a strong stress, such as a rubber band tourniquet, inhibits the stimulatory effect of PB on GH release as long as the stress is continued.