Cats anaesthetized with a mixture of chloralose and urethane were exposed to heat stress in two groups. In the first group (n = 10) of control animals, the effect of heat stress on haemodynamic variables was recorded at control haematocrit (HCT) of 42.0 +/- 1.0%. In a second group, the effect of heat stress was studied after induction of acute normovolaemic haemodilution (HCT of 13.0 +/- 1.0%). Haemodilution was induced to a maximum of 60% replacement of blood with dextran (mol.wt. 150,000). Heat stress was induced by surface heating and core body temperature was raised from 37 degrees C to 42 degrees C. The effect of heat stress and haemodilution on various haemodynamic variables, viz. left ventricular pressure (LVP), left ventricular contractility (LVdP/dtmax), heart rate (HR), cardiac output (CO), arterial blood pressure (ABP), right atrial pressure (RAP), and arterial blood PO2, PCO2 and pH was examined. Haemodilution produced significant (P < 0.05) increases in HR and CO but there were no significant (P > 0.05) changes in ABP, RAP, LVdP/dtmax and total peripherial resistance (TPR). Hyperthermia caused a significant fall (P < 0.05) in TPR. However, the percentage fall in TPR was higher in the control group. On exposure to heat stress, there were significant (P < 0.05I increases in HR and CO in both the groups; however, HR and CO values were significantly (P < 0.05) higher in the haemodiluted group compared to the control. The latter findings could be due either to the higher basal values of these variables with the fall in HCT or to inefficient cardiovascular regulatory mechanisms. The lack of efficient regulatory control under such severe stress conditions makes the cardiovascular system of anaemic animals more vulnerable to heat stress. In conclusion, the results of the present study showed deleterious effects of heat stress in both the groups. The higher values of HR and CO in the haemodiluted group may be responsible for circulatory failure at low HCT values, indicating a higher risk in the haemodiluted group as compared to the control group.