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The rate of TTC reduction by the uterine tissue of rats was determined at specific stages of the oestrous cycle, and in successive cycles at the same stage. Individual females showed a characteristic level of reaction to TTC. Mothers whose uteri had a high reaction rate with TTC produced litters with a higher percentage of female pups than did mothers with a low level of reaction. Previous studies have shown that TTC reduction rate represents a metabolic activity of uterine tissue that is induced by oestrogens.