AbstractPrior research has found that mice reared and nursed by a lactating rat mother, or reared in the presence of a nonlactating adult female rat, give a significantly smaller adrenocortical response to a novel stimulus than do control mice. The purpose of this paper was to determine whether this phenomenon could be mediated through stimuli that do not require physical contact between the adult rat and the young mouse. Experimental mice were born and reared in a cage that separated them from an adult nonlactating female rat by means of a double mesh wall. Thus, they could see, hear, and smell the rat, but no physical contact was possible. Control mice were reared similarly in the presence of an adult nonlactating female mouse. At the time of weaning the corticosterone response of both groups to a novel stimulus was virtually identical. The data support the hypothesis that direct physical contact between the mouse and rat is necessary to cause physiological and behavioral changes in the mouse.