Diuretics, in particular furosemide, are generally recommended as a supportive treatment in the advanced stages of heartwater in ruminants. However, after what appeared to be possible adverse effects accompanying its use in field cases of heartwater, the effects of this drug on certain blood and urine parameters were investigated in normal sheep at the same dose rates. Diuresis with concomitant natriuresis was significant after furosemide administration, as was the expected plasma volume decrease. Other significant changes included metabolic alkalosis, hypokalaemia and reduced blood ionised calcium. The difference in duration of the diuretic effect and the duration of the changes in blood parameters from c. 3 h and c. 6 h respectively make it difficult to determine a time interval between successive treatments with furosemide. It appears that the probable cause of death of sheep with heartwater is a drastic reduction in blood volume and decreased cardiac output that leads to general circulatory failure. A therapeutic approach that involves further loss of plasma volume due to diuresis appears contradictory. The added effects of potentiating respiratory alkalosis and the terminal drop in blood ionised calcium seen in heartwater-affected animals indicate that the use of furosemide in supportive treatment of this disease is not warranted.