When rodents are placed on an elevated plus-maze which consisted of two enclosed arms and two open arms, they enter the enclosed arms more frequently and spend more time on the enclosed arms than on the open arms since they prefer the enclosed arms to the open arms. The elevated plus-maze test can measure effects of drugs based on this tendency of rodents. Anxiolytics increase the time spent on the open arms and number of entries into open arms, and anxiogenics decrease them. We investigated the basal behavior of mice and the effects of diazepam in the elevated plus-maze test using 4 mazes with different size. The basal behavior of mice were changed by the maze size, especially by the width of the open arms. Further, we suggested that the maze size might affect anxiolytic activity of drugs detected in this test. This test is very useful because it needs neither complicated training for rodents nor an expensive apparatus, and the anxiolytic effects of drugs can be easily measured. However, this test is difficult to detect putative anxiolytics acting on serotonin receptors. When this method is used for measuring the activity of drugs, the choice of experimental conditions and evaluation of the results should be carefully done.