As only one experimental study examining the effects of haemodilution on circulatory responses to hyperthermia has so far been published and there is no information on respiratory responses to hyperthermia during haemodilution or anaemia, this paper studied the effects of acute normovolemic haemodilution on cardiorespiratory changes during 42 degrees C hyperthermia and its recovery by body surface cooling in 16 anaesthetized adult rabbits. The animals were divided into two groups: haemodiluted group (Hct = 18.6 +/- 0.4%) and control group (Hct = 41.1 +/- 0.9%). In the haemodiluted group, acute normovolemic haemodilution was induced by 60% replacement of total blood volume with dextran. Haemodilution produced significant increases in minute volume (V(E), p < 0.02), heart rate (HR, p < 0.02) and central venous pressure (CVP, p < 0.02), but there was no significant change in mean arterial pressure (MAP). Hyperthermia caused significant increases in V(E) and HR in both the groups; however, V(E) and HR values were significantly higher in the haemodiluted group compared to the controls. CVP decreased (p < 0.05) during 42 degrees C hyperthermia only in the controls in the panting phase. MAP did not significantly change during over-heating in controls, but it significantly (p < 0.02) rose in the haemodiluted group. In the recovery phase, cooling led to significant decreases in HR and MAP in both the groups and to further increases in V(E) (p < 0.05) in controls, whereas V(E) decreased in the haemodiluted animals. There were found no significant cardiorespiratory differences between the two groups during cooling. Hyperthermia was accompanied by the higher values of minute volume and heart rate in the haemodiluted animals, indicating a greater activation of the respiratory and cardiovascular systems, which could result in diminished functional cardiorespiratory reserve and a higher risk of respiratory and circulatory failures in haemodiluted/anaemic animals during hyperthermia. In the phase of recovery of the body temperature there were no significant cardiorespiratory differences found between the two groups.