To investigate the stereospecificity of beta-adrenoceptor blockade and renal vasodilatation by tertatolol, the regional hemodynamic effects of the 2 stereo-isomers of tertatolol (d-tertatolol and l-tertatolol) were compared in conscious spontaneously hypertensive rats. Furthermore, the effects of a combination of d-tertatolol and dl-propranolol were studied. l-Tertatolol and the combination of beta-blockers lowered heart rate and caused an early increase and a late decrease of blood pressure. l-Tertatolol, but not the combination of dl-propranolol and d-tertatolol, decreased renal resistance at 3-4 hr after injection. Injections of d-tertatolol alone did not cause any effect. Since effects of l-tertatolol are fully comparable to effects previously observed with dl-tertatolol, and d-tertatolol was devoid of any effects, both alone and in combination with dl-propranolol, we conclude that both beta-adrenoceptor blockade and renal vasodilatation are stereospecific for the l-isomer of tertatolol.