The role of endothelium-derived contracting factor or factors in modulating relaxations and contractions to adenine nucleotides was examined in aortas from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) and Wistar rats. During contractions to phenylephrine, the relaxations to ATP were impaired significantly in SHR compared with WKY aortas with endothelium. In rings treated with NG-nitro-L-arginine (to inhibit nitric oxide synthase), the endothelium significantly augmented contractions evoked by ATP; this enhancement was greater in SHR compared with WKY aortas. Indomethacin (inhibitor of cyclooxygenase) and SQ 29,458 (antagonist of thromboxane/prostaglandin endoperoxide receptors) but not dazoxiben (inhibitor of thromboxane synthase) significantly augmented the maximal relaxation in WKY rats, abolished the impairment of the relaxation in SHR, and prevented the potentiation by the endothelium of the contractions evoked by ATP. In older animals (10 to 12 months old), the endothelium-dependent concentration-relaxation curves to ATP in SHR and WKY aortas treated with indomethacin were superimposable, as were the concentration-contraction curves (with NG-nitro-L-arginine present). Endothelium-dependent concentration-relaxation and -contraction curves to ADP obtained in these preparations overlapped also. In Wistar rats, the magnitude of the endothelium-dependent relaxations to either ATP or ADP were significantly smaller compared with the other strains, and the endothelium-dependent contractions were even smaller. Results show that adenine nucleotides stimulate the production of both endothelium-derived relaxing and contracting factors. Although there is no obvious age-related alteration in the capacity of aortas to release endothelium-derived relaxing factor, aging enhances endothelium-derived contracting factor activity in WKY rats.