Human albumin 5% (HA), frequently used in paediatric anaesthesia as a human plasma substitute, could be replaced by hydroxyethyl starch 6% (HES) 40,000/0.5 provided its use would not entail any disadvantages but rather advantages instead. This problem was studied by examining 30 children (mean body weight 32.5 kg) in general anaesthesia. During about 3 hours of surgery the patients lost up to 15 per cent (approx. 400 ml) of blood volume. In a randomised study the blood loss was compensated either via HA or HES with 14 ml/kg body weight each, respectively. Blood pressure and pulse rate remained within the normal range. Haemodilution was represented by a drop in the erythrocyte count to about 3 million/microliter, haematocrit (HCT) to about 30% and haemoglobin to approx. 10 g/100 ml. Slight metabolic acidosis (Bodansky unit = -4 mmol/l) was found to be statistically not significant (= n.s.) with both methods, as was the difference in serum albumin and during serum electrophoresis. Serum [Na+] was reduced in those children who had been treated with HES, to 137.33 +/- 33.30 mmol/l; however, in those children who received HA with low sodium content (statistically significant difference = s.s.) the corresponding level was 134.15 +/- 2.36 mmol/l. Serum creatinine rose in each case from 60 to 80 mmol/l (s.s.), renal function being slightly impaired probably due to the anaesthesia and surgery. The value according to Quick's test and the partial thromboplastin time (PTT) remained in the normal range both with HA and HES treatment (in each case over 70% and below 25s, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)