The extent of stimulation of the enzyme adenylate cyclase, and the concomitant production of cAMP, by a number of beta-adrenoceptor agonists, all belonging to the class of the N-tert-butylphenylethanolamines, has been determined. The results have been used as direct measures for intrinsic sympathomimetic activity (ISA) and were correlated with various physicochemical parameters of the compounds. Significant correlations were established by means of the method of multiple regression analysis, and it was demonstrated that electronic effects only govern ISA. The use of 13C NMR chemical shifts of the aromatic C atoms proved to be a valuable tool in this analysis.