Ethinyloestradiol and progesterone affect monoamine metabolism in mouse brain. Previous investigations have suggested that the hormones may affect the release or re-uptake of the monoamines; this experiment was intended to give preliminary information on the effects of the hormones on monoamine synthesis. Four groups of 5 ovariectomised mice were injected with tetrabenazine (100 mg/kg, i.p.). 12 hrs later, one group was injected with iproniazid (100 mg/kg, i.p.), one with iproniazid and ethinyloestradiol (25 ~g/kg, s.e.), and one with iproniazid and progesterone (2.5 mg/monse, s.c.). All mice were killed 4 hrs after the iproniazid injection (16 hrs after tetrabenazine) and noradrenaline and 5-hydroxytryptamine concentrations were determined in three regions of the brains: the fore-brain (cortex, hippoeampns), the middle-brain (hypothalamus, thalamus, striatum), and the hindbrain (midbrain, pons, medulla, cerebellum). Ethinyloestradiol increased the accumulation of noradrenaline in the fore-and middle-brain regions; progesterone decreased the accumulation of noradrenaline in the fore-and middle-brain regions and of 5-hydroxytryptamine in the middle-and hind-brain regions.