Heat shock proteins (in particular Hsp70) and other components of this system were investigated in the nucleated erythrocytes of toads that are ectothermic and are exposed to a wide range of ambient temperatures. We have studied the heat shock proteins in two species that differ profoundly in their sensitivity to low temperatures, a cold resistant (Bufo viridis) and a cold sensitive (Bufo regularis) toad. Using mammalian anti Hsp70 (Sigma) on Western blot it was found in more than 20 specimens of each species that Hsp70 in red blood cells of the two species is expressed constitutively at high levels in control, room temperature conditions. It was expressed variably and comparable in the two species following thermal stress. The formation of HSF1-HSE binding complex, which is required for the expression of the stress protein, was unconnected to thermal conditions. Although the toads possess the components involved in the cellular stress response, the relationships and regulation of this system seem to be substantially different from the regular response of this system in mammals. It is hypothesized that the high resting level of Hsp70 in toad erythrocytes provides the basis for the survival in the ambient stress conditions. This hypothesis was tested on blood of two examplary species, an ectothermic lizard (Agama stelio) and an endothermic chicken (Gallus domesticus). In the chicken blood Hsp70 responded directly and reversibly on temperature change, whereas the lizard responded similar to the toads. It is suggested that a cellular strategy that involves a permanently high level of Hsp70 is predominant in vulnerable species, and makes them prepared to unpredictable stress stimuli that might interfere with appropriate cellular functions in the organism.