The open field test is used to assess ambulation and anxiety; one way to assess anxiety is to compare ambulation in the center with ambulation in the periphery: the more anxious is the mouse, the less it moves in the center. The results of this report cast doubts on the generality of that rule, because they show that ambulation, both in the center and in the periphery, depends on the mouse strain and on the size of the open field; specifically, in a brightly lit open-field of moderate size (38 x 31 x 25 cm), ambulation in the center reflects anxiety in Balb/c mice, but not in C57Bl/6 mice. Yet, a large open-field (100 x 100 x 30 cm), receiving approximately the same amount of light as in the mouse room, allows assessment of anxiety and exploration-driven ambulation in both strains of mice. To do that, the author of this report proposes (i) to express ambulation in normalized scores (i.e., ambulation per surface unit) to verify that ambulation in the periphery is higher than ambulation in the center, (ii) to use an open field sufficiently large so that mice of any strain ambulate more in the periphery than in the center, and (iii) to measure ambulation in concentric strips of the open field and plot ambulation against the distance of the strips from the wall: it is proposed that the intercept of the line reflects explorationdriven ambulation whereas the slope reflects fear of leaving the wall.