46Sc-and 99mTc-labelled microspheres were used to measure the effects of noradrenaline infusion on cardiac output and its regional distribution in 10 control and 10 streptozotocin-diabetic pithed rats. Plasma noradrenaline concentrations during the infusion were similar in both groups. Pressor responses were significantly smaller in the diabetic animals (controls: + 79, diabetic: + 44 mmHg; p less than 0.001). Cardiac output remained similar in both groups before and during the noradrenaline infusion. Total peripheral resistance was similar in both groups before noradrenaline but the noradrenaline-mediated increase was significantly smaller in the diabetic animals (controls: + 150%, diabetic: + 76%; p less than 0.05). Noradrenaline-mediated resistance increases were significantly reduced in several tissues of the diabetic rats including the small intestine (controls: + 132%, diabetic: -4%; p less than 0.005), the large intestine (controls: + 150%, diabetic: + 39%; p less than 0.05) and the kidneys (controls: + 180%, diabetic: + 27%; p less than 0.05), but were very similar in other areas, e.g. in the hindlimbs and tails.