A cardiac stress was performed in the isolated perfused rabbit heart by means of four injections of 0.5 micrograms epinephrine in the coronary vessels. The endogenous (-)carnitine/acetyl(-)carnitine ratio was also determined. The two exogenous optical isomers of carnitine possess a different pharmacological behavior on the epinephrine-induced stress. Exogenous (-)carnitine reduces the epinephrine stimulating effect whereas exogenous (+)carnitine increases it, including coronary flow. Furthermore, the endogenous (-)carnitine restores this ratio to the normal value, whereas exogenous (+)carnitine does not. It is concluded that carnitine might represent an important natural way for organism safeguard in the epinephrine stress as occurs in various emotional states. In this respect the importance of exogenous (-)carnitine as a therapeutic means remains to be investigated.