In order to investigate the immediate (i.e. within 3 h) response of albumin synthesis to the administration of endotoxin, as a model of a moderate and well controlled catabolic insult, two measurements employing L-[ 2 H 5 ]phenylalanine were performed in 16 volunteers. One group (n l 8) received an intravenous injection of endotoxin (4 ng/kg ; lot EC-6) immediately after the first measurement of albumin synthesis, whereas the other group received saline. A second measurement was initiated 1 h later. In the endotoxin group, the fractional synthesis rate of albumin was 6.9p0.6 %/day (meanpS.D.) in the first measurement. In the second measurement, a significant increase was observed (9.6p1.2 %/day ; P 0.001). The corresponding values in the control group were were 6.6p0.6 %/day and 7.0p0.6 %/day respectively (not significant compared with first measurement and P 0.001 compared with the second measurement in the endotoxin group). The absolute synthesis rates of albumin were 148p35 and 201p49 mg : kg − 1 : day − 1 before and after endotoxin (P 0.01). In the control group, the corresponding values were 131p21 and 132p20 mg : kg − 1 : day − 1 (not significant compared with the first measurement and P 0.01 compared with the second measurement in the endotoxin group). In conclusion, these results indicate that albumin synthesis increases in the very early phase after a catabolic insult, as represented by the administration of endotoxin.