Bupranolol is an antagonist at the cardiostimulatory low-affinity state of b(1)-adrenoceptors and we were interested whether this effect is shared by its fluorine (GD-6), methyl (DZ-51) and isopropyl analogue (DZ-13) and by the analogue hydroxylated at the tertiary butyl moiety (DZ-52). (-)-Bupranolol and compounds (-)-GD-6, (+)-GD-6, (-)-DZ-13, (+)-DZ-13, DZ-51 and DZ-52 antagonized the CGP 12177-induced tachycardia in pithed rats with "apparent pA(2) values" of 6.1, 6.1, 4.6, 5.5, 4.6, 5.1 and 5.3, respectively. For comparison, their potencies and affinities at the high-affinity state of b(1)-adrenoceptors were determined, using the xamoterol-induced tachycardia in pithed rats and [(3)H]CGP 12177 binding to rat cerebrocortical membranes. The respective "apparent pA(2) values" in the functional experiments were 7.9, 8.1, 5.4, 8.4, 5.7, 7.3 and 6.8 and the pK(i) values in the binding experiments were 8.8, 8.4, 6.9, 8.5, 6.7, 8.4 and 8.2. In conclusion, (-)-bupranolol and its fluorine analogue (-)-GD-6 are equipotent at the low-affinity state of beta(1)-adrenoceptors. The stereoselectivity of GD-6 and DZ-13 suggests that the low-affinity state is indeed a receptor.