Pregnancy-block after exposure of recently inseminated females to strange males was studied in mice of the Q strain. In one series of experiments the incidence of blocked pregnancy was very low. This strain displayed a normal increase in the frequency of oestrus when the females were exposed to males, hence the low pregnancy-block rate could not be attributed to anosmia. In other experiments, Q females displayed a high incidence of pregnancy-block after a period of adaptation. The initial low

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... te of pregnancy-block was evidently due to disturbance resulting from unaccustomed daily handling during the test.