Heat shock proteins were investigated in nucleated erythrocytes, skeletal and heart muscles of toads that are ectothermic and are exposed to a wide range of ambient temperatures. Two species that differ profoundly in their sensitivity to low temperatures, a cold resistant (Bufo viridis) and a cold sensitive (Bufo regularis) toad were investigated by Western blotting. It was found that Hsp70 in those tissues in the two species is expressed constitutively at high levels in control room temperature conditions. 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 toads' tissues provide the basis for the survival in the changing ambient stress conditions. This hypothesis was tested on blood of two exemplary species, an ectotherm lizard (Agama stelio) and an endotherm chicken (Gallus domesticus). In the chicken blood Hsp70 responded directly and reversibly to temperature changes, whereas in the lizard the response was similar to that in the toads. It is suggested that a cellular strategy that involves a permanently high level of Hsp70 is predominant in vulnerable poikilothermic species and makes them prepared to unpredictable stress stimuli.