The present study is concerned with the demonstration of the influence of long term treatment with caffeine and phenobarbital on pentobarbital sleeping time, gastric secretion, increase rate of body weight and brain and liver weight in rats. The experimental subjects were rats weighing about 140 to 150 g, each of them was isolated in a separate cage. Each group was given 1 ml normal saline solution as control, caffeine 10 mg/kg and phenobarbital 30 mg/kg as experimental groups. All drugs were injected intraperitoneally, daily for 4 weeks. The results obtained are summarized as follows; 1. There was significant difference between before and after injection of drugs (caffeine citrate 10 mg/kg and phenobarbital 30 mg/kg) on pentobarbital sleeping time. The sleeping time of caffeine treated group was delayed (22.4%, p<0.01) significantly compared with that of before injection. The sleeping time of phenobarbital treated group was markedly shortened (93.6%, p<0.001) compared with that of before injection of drugs. 2. The volume, free and total acidity and pH of gastric juice determined five hours after pyloric ligation in fasting rats were not significantly changed in experimental groups compared with control group. However the volume of gastric juice was increased 25% in both caffeine and phenobardital treated group. 3. The increased ratio of body weight revealed no remarkable difference compared with intial body weight. However, caffeine treated group showed markedly increased body weight after first and second week of injection. 4. The brain and liver weight in experimental group showed no significant difference compared with control group (as percentage of body weight).