Genetic and nongenetic sources of variation in the weight response of the immature rat ovary to a gonadotrophic hormone.

COMMON method of bioassay of gonad-stimulating preparations is A based on the gain in the ovary-weight of the immature female rat in relation to the amount of injected material. The large variability in the response of the ovaries of different rats of the same age to similar treatment is common knowledge of endocrinologists. The exact sources of this variability and their relative importance, however, are not common knowledge, even though some of them, such as strain and body weight, are sometimes considered in designing experiments or comparing experimental results. The results reported here indicate the extent to which some of the factors influence this variability of stimulated ovary weights in a particular colony of rats. They stress the need for cognizance of these elements of variability in designing experiments to gain more efficient use of animals and to interpret results of experiments with more precision.