1. Three daily sessions of inescapable footshock stress of 30 min duration each increased the sensitivity of the isolated pacemaker of the rat to the chronotropic effect of isoprenaline. 2. The effect of inescapable footshock stress on the sensitivity of the isolated rat pacemaker to the chronotropic effect of isoprenaline was prevented by the daily administration of the compound RU-38486, a potent antiglucocorticoid which blocks the cytosolic receptor for corticosterone. 3. The administration of the compound RU-28362. a potent agonist of the cytosolic receptor for corticosterone, during 3 days to rats which were not submitted to footshock stress induces supersensitivity to the chronotropic effect of isoprenaline. 4. It is concluded that corticosterone plays an important role in the qualitative control of the rat pacemaker beta-adrenoceptor population during adaptation to repeated footshock stress.234787791