The dose-response curves to the chronotropic effect of adrenaline obtained in right atria isolated from female rats indicated an order of increasing sensitivity to adrenaline, at the pD(2) level, according to the estrous cycle, as follows: estrus I metestrus I diestrus I proestrus. Inhibition of neuronal and extraneuronal uptake shifted the dose-response curves to adrenaline to the left only in right atria isolated from rats during estrus or metestrus. Moreover, under these experimental conditions, right atria were subsensitive to adrenaline during proestrus, in contrast to metestrus. Plasma corticosterone levels were lower during estrus and higher at proestrus. There was a positive correlation between right atria sensitivity to adrenaline and plasma corticosterone levels and estrous cycle phases. Our results also suggest that in the rat right atria during proestrus, as opposed to the other phases of the estrous cycle, there was an endogenous inhibition of extraneuronal uptake together with some alteration at the adrenoceptor level and (or) at intracellular mechanisms beyond receptors.735602607