Summary:The cardiac beta adrenergic receptor blocking and the negative isotropic activities of tiprenolol [DU21445 1—(isopropylamino) ‐3‐ (0‐methyl thio phenoxy)‐propanol‐2, hydrochloride. (N.V. Philips‐Duphar, Weesp, The Netherlands)] were compared with those of propranolol by studying the conventional haemodynamic measurements and systolic time intervals at rest and during exercise in ten patients with ischaemic heart disease. Two tests were done on separate days each with a different beta adrenergic blocking drug given in equipotent doses with respect to exercise heart rate. The sequence in which the drugs were given was unknown to the investigating team and each test was controlled with a placebo unknown to the patient. The beta blocking drugs were given in equipotent doses with regard to exercise heart rate. Tiprenolol 2 mg was shown to have significantly less heart rate slowing effect at rest than propranolol 6 mg (P<0.05). However, all other measurements failed to show any difference of statistical significance between the two drugs with respect to any negative inotropic or beta‐bl