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ONE FIGUREThe length of heat in the rat, a s in the guinea pig, is a measure of reproductive normality which has not been widely used. I n most morphological investigations the vaginal condition has served as the end-point and in most studies of behavior (Hemmingsen, '33; Ball, '34, '36, '37 and '40) reliance has been placed on the intensity of the copulatory response. Nevertheless, the course of heat can easily and accurately be followed and once its length has been ascertained a very satisfactory basis has been established for determining and studying individual differences and f o r investigating factors involved in the induction and control of mating behavior (Boling and Blandau, '39). As matters stood, however, there was a need f o r new determinations because no two of the earlier estimates were the same. Tshii ( '22) gives the length as 4 to 8 hours, Long and Evans ( '22) as 3 to 1 2 lioui-s, and Ball ('37) as 3 to 21 hours.There were other reasons for an interest in the problem. When Ball ('37) was studying the rat with much the same purpose we have had, she was troubled by the tendency of animals tested a t frequent intervals to develop a "disturbed endocrine balance" which prolonged the cycle and period of comification and modified the behavioral picture. She states that discontinuance of the practice of making vaginal smears