The chronotropic responsiveness to norepinephrine (NE) and isoproterenol (ISO) was determined in right atria isolated from rats submitted to repeated escapable or inescapable foot-shock. Significant postjunctional supersensitivity to ISO, but not to NE, was observed in both groups. No significant change in the pA2 value of metoprolol (a selective beta1-adrenoceptor antagonist) was detected. However, a decrease of the maximum response to soterenol, a partial agonist at beta1-adrenoceptors, occurred only after inescapable foot-shock. The enhanced sensitivity to ISO was abolished by butoxamine (a selective beta2-adrenoceptor antagonist) and accompanied by a marked increase in the pA2 Value of this antagonist. We conclude that the ability to control the shock prevented the down-regulation of the pacemaker beta1-adrenoceptors but not the increased participation of beta2-adrenoceptors in the response of the rat sinoatrial node to catecholamines after repeated foot-shock.444869875