When in the hypoxic pressure chamber and under high-altitude conditions, rats showed changes in the lipid profile of plasma membranes and in the activity of placental phospholipase A2. More profound changes occurred in the placental membranes of the rats staying under hypoxic conditions during placentation. Placental plasma membranes in the animals exposed to hypoxia in the second gestational half were more stable, their lipid composition was less altered.