1. To investigate the structure-activity relationships of alpha-adrenoceptor agonists for the alpha 1-adrenoceptor subtypes, we have compared the imidazoline class of compounds, oxymetazoline and cirazoline, with the phenethylamine, noradrenaline, in their affinities and also in their intrinsic activities in Chinese hamster ovary (CHO) cells stably expressing the cloned human alpha 1-adrenoceptor subtypes (alpha 1a-, alpha 1b-, and alpha 1d-subtypes). 2. Radioligand binding studies with [125I]-HEAT showed that cirazoline and oxymetazoline had higher affinities at alpha 1a-subtype than at alpha 1b- and alpha 1d-subtypes, while noradrenaline had higher affinity at the alpha 1d-subtype than at alpha 1a- and alpha 1b-subtypes. 3. In functional studies, cirazoline caused transients of cytosolic Ca2+ concentrations ([Ca2+]i response) in a concentration-dependent manner and developed a maximal response similar to that to noradrenaline in CHO cells expressing the alpha 1a-subtype, while it acted as a partial agonist at alpha 1b- and alpha 1d-adrenoceptors. Oxymetazoline, on the other hand, was a weak agonist at alpha 1a-adrenoceptors, and has no intrinsic activity at the other subtypes. 4. Using the phenoxybenzamine inactivation method, the relationships between receptor occupancy and noradrenaline-induced [Ca2+]i response for alpha 1a- and alpha 1d-subtypes were found to be linear, whereas it was moderately hyperbolic for the alpha 1b-subtype, indicating the absence of receptor reserves in CHO cells expressing alpha 1a- and alpha 1d-subtypes while there exists a small receptor reserve for CHO cells expressing the alpha 1b-subtype. 5 In summary, our data obtained in cells exclusively expressing a single receptor subtype support the idea that the relative role of agonist affinity and intrinsic activity may vary depending on the subtype of alphal-adrenoceptor.