TA%EMOTO, T., SUZUKI, T. and MIYAMA, T. Effects of Isolation on Mice inRelation to Age and Sex. Tohoku J. exp. Med., 1975 , 117 (2), 153-165-Isolated housing conditions (a single mouse in cage) have been reported to induce various changes in physiological and behavioral characters of mice comparing with collective-housing conditions. However, the effect of age and sex has not been studied in detail.In this report, the young (8 or 11 weeks old) and the mature (29 weeks old for male and 32 weeks for female) of both sexes were comparatively studied on the response to isolation.The duration of isolation was 12-13 weeks. Significant differences were seen in body weight, weights of fur and interscapular adipose tissue, and weight of adrenals in male, and the difference in weight of adrenals was more outstanding in the young than in the mature . In female, only weight of ovaries differed significantly between the isolation and the collective groups, and the difference prevailed in the young than in the mature.The heavy body weight with fat deposition and small adrenals in the isolated male is not consistent with the results on the isolated male rat (Hatch et al. 1965). Small ovary in the isolated female is a similar finding to reports on mice (Weltman et al. 1962) and on rats (Hatch et al. 1965). isolated housing; space of cage; adrenal weight; ovarian weight; fat depositionThe housing condition of experimental animals has raised a topic in these years. Many researchers reported that isolated animals showed various physiological and behavioral peculiarities or different susceptibilities to drugs or environmental stress comparing with those reared in the collective condition (King et al. 1955;Barnes 1958Barnes , 1959Harry and Harlow 1962;Weltman et al. 1962;Barrett and Stockham 1963;Hatch et al. 1963Hatch et al. , 1965Thiessen 1963;Jewett and Norton 1964;Wiberg and Grice 1965;Krech et al. 1966;Wiberg et al. 1966;Agrawal et al. 1967;Baumel et al. 1969;Bernstein and Moyer 1970;Brain 1971Brain , 1975Brain et al. 1971;Takemoto et al. 1975).As one of these peculiarities due to prolonged isolation, King et al. (1955) observed the convulsion in the singly caged CH mice. The present authors reported different incidence rates of convulsive attacks according to different conditions of isolation in male dd-D mice (Takemoto et al. 1975).