This test is based on conflicting innate tendencies of avoidance of bright light and open spaces (that ethologically mimic a situation of predator risk) and of exploring novel environment. When placed into a brightly lit open field for the first time, rats and mice tend to remain in the periphery of the apparatus or against the walls (thigmotaxis). It had been shown that anxiolytics administration increases exploration time in the center of the open field while stressful stimuli decrease the number of center visits. Open field activity, therefore, represents a valid measure of marked changes in “anxiety-like” behaviors in drug-treated and genetically manipulated animals. Open-field procedure can also be used for general assessment of animal basal locomotor activity and exploration. In these cases, non-stressful conditions are needed (habituation, low lighting conditions) and the experiment duration can be longer.