1. The metabolism of 5-hydroxytryptamine (5-HT) in homogenates of lung and in perfused isolated lungs, was measured, using [14C]5-HT, over the 4-day oestrous cycle in rats. 2. In lung homogenates, 5-HT metabolism (reflecting monoamine oxidase (MAO) activity) was greater during met-oestrus than during the rest of the cycle. 3. In homogenates, the apparent Km for MAO activity was lowest during met-oestrus whereas the, Vmax value was highest during di-oestrus. 4. In perfused lungs, 5-HT metabolism and uptake was greater during pro-oestrus than during met-oestrus. 5. In lung homogenates prepared from female rats treated with 17 beta-oestradiol or progesterone for 8 days, the metabolism of 5-HT was increased. 6. We conclude that although the lung is not usually considered a target organ for ovarian hormones, its metabolic activity is affected by changes in the levels of endogenous or exogenous ovarian steroids, an effect comparable with those already observed in uterus, ovary and other more commonly accepted target organs.