Organic nitrates exert their effect by direct inhibition of vascular tone, resulting in decreased myocardial oxygen demand and improved oxygen supply. Less well known is the effect of nitrates on blood rheology. To evaluate the haemodiluting effect of intravenous nitroglycerin we studied 70 consecutive patients admitted to our coronary care unit. The study group consisted of 51 patients with acute chest pain who were treated with nitroglycerin intravenously in a dose between 1 and 5 mg.h-1 during the first 24 h of admission; 19 patients without this treatment were used as controls. In the study group, values of haemoglobin, haematocrit, serum protein and serum albumin, as indirect parameters of haemodilution, all decreased significantly within 24 h, irrespective of the final diagnosis. Similar values were found in the control group. The mean decrease in the haemoglobin value was 0.92 (SD = 0.55) mmol.l-1 vs controls -0.05 (SD = 0.48) mmol.l-1, P < 0.05; the mean decrease in haematocrit was 0.047 (SD = 0.032) vs controls 0.000 (SD = 0.023), P < 0.05. Similar decreases were found in serum protein and albumin values. It is concluded that intravenous nitroglycerin causes a significant decrease in haemoglobin and haematocrit values within 24 h of administration. This altered hemorheologic state may, besides the well known direct vascular effects of nitroglycerin, contribute to the protective effect of nitroglycerin in acute myocardial ischaemia. The effects of nitroglycerin, and the possible mechanisms underlying the decrease in haemoglobin and haematocrit values, will be discussed.