Various steroids (17-beta-oestradiol, corticosterone, deoxycorticosterone, progesterone, testosterone and androsterone) produced a dose-dependent inhibition of the uptake of (3)H-noradrenaline by the Uptake(2) mechanism in the isolated perfused heart. It is suggested that these results may explain the potentiating effects of such steroids on the responses of vascular smooth muscle to catecholamines.