We have examined the relaxation produced by the β-adrenoceptor agonists isoprenaline and procaterol, and by sodium nitroprusside, forskolin and isobutyl-methylxanthine in KCl (40 mmol/1)-contracted aortae from 6 week, 3 month and 6 month old Sprague-Dawley rats. The maximum relaxation to all agents was greatest in aortae from 6 week old animals. The relaxation to the β-adrenoceptor agonists was virtually abolished, and the relaxation to the other agents significantly reduced, in aortae from 3 and 6 month old animals. There were no significant differences in the response to any agent between aortae from 3 and 6 month old animals, so that the changes seen occurred in maturation from 6 week to 3 month. The reduced vasodilator response does not occur at the level of the β-adrenoceptor or adenylate cyclase, since the response to sodium nitroprusside was also affected, but is a general reduction in the ability of the aorta to relax.