The oestrous cycle and monoamine oxidase activity.

1. During the oestrous cycle of the rat, monoamine oxidase (MAO) activity was studied in the uterus, the ovaries and the adrenal glands and also in 4 brain regions which are rich in monoamines. In all these structures the activity was lowest during the night between oestrus and metoestrus. High values were found in di-oestrus, with the exception of the uterus in which the MAO activity reached its peak during metoestrus.2. A stimulating effect of progesterone on uterine, ovarian and adrenal MAO activity was observed.3. An index for the cyclic variations in progesterone production was obtained by measuring the adrenal and ovarian progesterone content at the same stages of the cycle as MAO activity.4. The sum of the amounts of progesterone contained in the ovaries and adrenal glands was low in early oestrus but increased during late oestrus when MAO activity was at its minimum. Progesterone contents were high in met- and di-oestrus when MAO activity was also high.