In 60 mothers with normal placental function, the relative merits of right and left lateral tilt during Caesarean section were assessed. Maternal-foetal biochemical values and relationships were generally more satisfactory with the left lateral position. The incidence of hypotension (revealed aorto-caval occlusion) before and after induction of anaesthesia, was significantly greater (P less than 0.001) with the "right side down" posture.