Isolated arteries from the femoral circulation of Wistar rats mounted on a small vessel myograph demonstrated age related tension development to noradrenaline (NA, 1 x 10(-8) -5 x 10(-5) M) day 20 greater than day 10 (P < 0.005); day 100 greater than day 20 (P < 0.001) and depolarizing potassium (125 mM) buffer day 20 greater than day 10 (P < 0.001). NA evoked dilatation in femoral arteries from neonatal rats (10 days) when added to unstimulated vessels or to those preconstricted with the thromboxane mimetic, U46619. Relaxation to NA was inhibited by L-NAME (0.1 mM) (P < 0.001), endothelial removal (P < 0.001) and the alpha(2)-adrenoceptor antagonist, yohimbine (0.1 mu M) (P < 0.001). alpha(1)- or beta-adrenoceptor antagonism was without effect. Relaxation was evoked in femoral arteries of the 10-day-old rats by the alpha(2)-adrenoceptor agonist UK14304 (1 x 10(-8) -5 x 10(-5) M). This relaxation was also abolished by L-NAME (0.1 mM) (P < 0.001) or endothelial removal (P < 0.001). a,adrenoceptor-mediated vasorelaxation was the predominant response to NA stimulation in femoral arteries of the neonatal rat. These responses were endothelium-dependent and were NO-mediated.