1. Isoprenaline (3 x 10(-8)-10(-5) M), salbutamol (3 x 10(-7)-10(-4) M) and forskolin (3 x 10(-9)-3 x 10(-7) M) relaxed rat isolated thoracic aortic rings contracted with noradrenaline (10(-7) M). Removal of the endothelium from the aortic rings abolished the effect of acetylcholine (10(-6) M) and completely prevented the vascular relaxation induced by isoprenaline, salbutamol or forskolin. 2. The isoprenaline concentration-relaxation curve was shifted in parallel to the right about 10 fold by propranolol (3 x 10(-7) M) with no change in the maximum response, showing that the relaxation was mediated by a beta-adrenoceptor. 3. The inhibitor of nitric oxide synthesis, NG-nitro-L-arginine (L-NOARG; 10(-5) M), shifted the isoprenaline relaxation curve to the right and reduced the maximum response. 4. Isoprenaline (10(-6) M) relaxed noradrenaline-induced tone by approximately 95% and at the same time increased levels of adenosine 3':5'-cyclic monophosphate (cyclic AMP) 4 fold and guanosine 3':5'-cyclic monophosphate (cyclic GMP) 12 fold in the aortic rings. Sodium nitroprusside (3 x 10(-8) M) relaxed noradrenaline-evoked tone by 82% without changing levels of cyclic AMP but raised cyclic GMP 19 fold. 5. Forskolin (10(-7) M) relaxed noradrenaline-induced tone by approximately 41% and, like isoprenaline, increased levels of cyclic AMP (2.5 fold) and cyclic GMP (12 fold) in the aortic rings. 6. Removal of the endothelium abolished the relaxant effects of isoprenaline (10(-6) M) and also the associated accumulation of cyclic AMP and cyclic GMP.7. L-NOARG (10- M) inhibited the relaxant responses and accumulation of cyclic GMP induced by isoprenaline (10-6 M) and forskolin (10- M) without affecting the associated cyclic AMP accumulation.8. It is concluded that, in the rat aorta, isoprenaline acts through a P-adrenoceptor on the endothelium to raise cyclic AMP and that this may, directly or indirectly, release nitric oxide to evoke vascular relaxation via the increase in cyclic GMP. The importance of this novel transduction pathway for cardiovascular regulation remains to be determined.