This paper presents the effect of chronic administration of psychotropic drugs on rats. The experimental animals were litter mates (average initial body weight 47 ± 1.1g) whose mother were bred at our laboratory. Each litter mate was treated as one group. Control animals were treated with tap water and each experimental group was treated with caffeine citrate 0.1%, nialamide 0.1%, ethyl alcohol 2.5%, phenobarbital sod. 0.1%, diphenylhydantoin 0.1%, chlorpromazine 0.1%, reserpine 0.005%, diazepam 0.01%, chlorpheniramine 0.01% solutions respectively in drinking water over a period of ten weeks. All rats were allowed food and drinking water ad libitum. The mortality rate and the per cent increase of body weight were recorded weekly throughout the course of the experiment. The effects of above agents on the pentobarbital sleeping time, gastric secretion, and brain and liver weights were studied at the end of ten weeks treatment. The obtained results are summarized as follows: 1. Mortality rate was highest in the groups treated with phenobarbital and chlorpromazine respectively. Through the experimental period (ten weeks), the mortality rate was higher in earlier stage than in the later period. 2. During the period of prolonged administration of psychotropic drugs, only diazepam treated group showed remarkable difference in per cent increase of body weight from the control group of rats. 3. Acute treatment with psychotropic drugs delayed the onset of pentobarbital sleeping time. In contrast, the sleeping time was significantly shortened (p<0.001) when the rats were treated chronically with those agents. 4. The effects of chronic treatment with phenobarbital or diphenylhydantoin on the gastric secretion are as follows: the total acidity was remarkably decreased while the pH was increased. 5. The brain weight was significantly decreased in the ethyl alchol and in the chlorpheniramine treated groups, in the mean time, there was no change in liver weight treated with any psychotropic drugs.