Noradrenaline sensitivity (pD2-value) and agonist dissociation constant (pKA-value) were determined in the following 4 canine arteries: thoracic and abdominal aorta, mesenteric and renal. The experiments were done in the presence of beta- and alpha 2-adrenoceptor blockers, and inhibitors of neuronal and non-neuronal uptake of catecholamine. A positive correlation was found in the pD2- and pKA-values for noradrenaline (r = 0.976, P less than 0.05). The slope (0.92) was not significantly different from 1. The pD2- and pKp (negative log of dissociation constant of partial agonist)-values of clonidine, an alpha 1-adrenergic partial agonist, were also determined. There was a positive correlation between the pD2- and pKp-values (r = 0.974, P less than 0.05). Again, the slope (1.39) was not significantly different from 1. These results indicate that the regional difference in the pD2-values for noradrenaline and clonidine in canine arteries is due to the affinities to alpha 1-adrenoceptors and that there is a qualitative difference between alpha 1-adrenoceptor mechanisms in the arteries and veins. However, no positive correlation between the pD2-values for noradrenaline and clonidine was observed (r = 0.182, P greater than 0.05). Thus, it may be suggested that either the recognition sites for noradrenaline, clonidine and prazosin are different or else their mode of attachment to the same recognition site is not identical.