The dissociation constants (KA values) of l-norepinephrine (l-NE) and seven other agonists acting on alpha adrenergic receptors in rabbit aorta strips were determined by analysis of concentration-response data before and after fractional inactivation of receptors with Dibenamine. In experiments to determine KA values, propranolol was added to block activation of beta receptors and cocaine to block the neuronal uptake mechanism. The KA of l-NE and the KA of a second agonist, when determined on paired strips from the same aorta, were used to calculate the relative affinity and the relative efficacy (er) of the second agonist as compared to l-NE. The validity of the method used for determining KA and er values was supported by the following findings. 1) The dissociation constant (KB) of the competitive antagonist, phentolamine, determined with each of the agonists, was the same as that determined with l-NE. 2) The KA determined for l-NE was independent of the fraction of active receptors remaining (q) after pretreatment with different concentrations of Dibenamine. 3) The KB of phentolamine determined with l-NE as the agonist was the same before and after fractional inactivation of receptors. 4) After inactivation in paired strips by equal exposures to Dibenamine, the q value determined with each agonist was the same as that determined with l-NE. The mean KA value for l-NE was 3.39 +/- 0.15 X 10(-7) M. The mean relative affinities of the agonists for the alpha receptor were: l-NE, 1;L-EPINEPHRINE, 1.25; L-PHENYLEPHRINE, 0.200; L-norphenylephrine, 0.217; epinine, 0.136; dopamine, 0.0055; l-alpha-methylnorepinephrine, 0.095; dl-alpha-ethylnorepinephrine, 0.0048. The mean er of each agonist was not significantly different from that of l-NE, except for l-norphenylephrine with an e of 0.71, and dl-alpha-ethylnorepinephrine with an er of 0.41. The results are discussed from the standpoint of structure-activity relationships.