In order to investigate the immediate (i.e. within 3h) 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 = 8) received an intravenous injection of endotoxin (4ng/kg; lot EC-6) immediately after the first measurement of albumin synthesis, whereas the other group received saline. A second measurement was initiated 1h later. In the endotoxin group, the fractional synthesis rate of albumin was 6.9±0.6%/day (mean±S.D.) in the first measurement. In the second measurement, a significant increase was observed (9.6±1.2%/day; P <0.001). The corresponding values in the control group were were 6.6±0.6%/day and 7.0±0.6%/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 148±35 and 201±49mg·kg -1 ·day -1 before and after endotoxin ( P <0.01). In the control group, the corresponding values were 131±21 and 132±20mg·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.