Behaviors of mice (BALB/c and C57BL/6) in the forced-free situation (open field with a shelter) were investigated and compared with behaviors of rats (Tsukuba Emotional rats (FUJITA, 1988)). It was found that in mice, the forced-free situation resembled neither the free nor the forced situation, while it had resembled the free situation in rats. Mice seemed to fear also the shelter. For example, mice hesitated to enter the shelter and came back as soon as they entered it. If BALB is considered to be more emotional than C57BL, this strain difference corresponded to that of Tsukuba Emotional rats (THE and TLE) in this situation.