Recently inseminated female deer mice were exposed to changes in physical environment, including size of available space, newness of environment, and a stud or strange male in order to test the hypothesis that a strange male decreases the incidence of pregnancy in recently inseminated females (Bruce effect). The data confirm the Bruce effect but also indicate that changes in physical environment produced great effects on implantation in recently inseminated females.

The concentration of unbound corticosterone in mice exposed to the presence of a trained fighter is much greater if the mice have previously experienced physical defeat than if they have not. There is little difference in the concentration of the hormone between mice placed in the presence of a fighter, given a background of physical defeat, and mice actually attacked and defeated. Two possible categories of stimuli which could be responsible for hyperactivity of the adrenal cortex following defeat by another mouse are psychological and physical (for example, bite wounds); the former is apparently by far the more important under the conditions of these experiments.

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