Hypertension and vascular disease are common complications in autosomal-dominant polycystic kidney disease (ADPKD). The role of changes in morphology and reactivity of resistance vessels in this disease have not previously been studied. Mesenteric resistance arteries were dissected from 8- to 14-week-old heterozygous Han:SPRD polycystic kidney disease (PKD) rats, homozygous normal Han:SPRD littermates (HSPRD) and Sprague-Dawley rats (SD). The morphology, noradrenaline (NA) contractility, endothelium-dependent acetylcholine (ACh) relaxation before and after incubation with L<sup>G</sup>-nitro- L-arginine methyl ester (L-NAME), and endothelium-independent 3-morphollino-sydnonimine (SIN-1) relaxation were studied with the Mulvany-Halpern myograph. Blood pressure and morphology of vessels were the same in all groups of rats, apart from a slightly higher media/lumen ratio in heterozygous PKD rats (p < 0.05). Active wall tension and contractile sensitivity to NA were higher in both heterozygous PKD rats and HSPRD than SD rats (p < 0.05). The maximum endothelium-dependent relaxation rate was markedly decreased in heterozygous PKD (19 ± 9%) and HSPRD (34 ± 12%) compared to SD rats (75 ± 11%) (p < 0.05). After incubation with L-NAME, ACh-induced relaxation was significantly attenuated in SD rats, less attenuated in HSPRD, and not significantly changed in heterozygous PKD rats. SIN-1-induced endothelium-independent relaxation was similar in all three groups. In conclusion, hyperreactivity to NA and impaired endothelium-dependent relaxation were present in resistance vessels from Han:SPRD rats, especially in animals with PKD. These abnormalities in resistance vessels from PKD rats may be important for the development of hypertension and vascular disease.