The maximal contractile response to the prostaglandin endoperoxide H2 analogue U46619, prostaglandins E2, D2 and F2 alpha and the sensitivity of the superfused aorta preparation to these drugs (except PGF2 alpha) is greater in the male than the female rat. In contrast, gender differences were not observed in the response to noradrenaline of 5-hydroxytryptamine. In previous studies, testosterone unlike oestrogen or progesterone, increased the response of both rabbit and rat aorta to U46619. We conclude that prostaglandin receptors in rat thoracic aorta may be hormonally regulated.