['The purpose of this study was to determine whether an ultra-endurance event is a strong stimulus to increase albumin synthesis involved in the process of intravascular albumin mass increase associated with transient hypervolemia.', 'The albumin synthetic rate was measured in six young men, 3 d before (C) and on the 1st (R1) and 8th (R8) days of the recovery from an ultra-endurance trial (5 h daily for 4 d). Albumin fractional (FSR) and absolute (ASR) synthetic rate were determined using a primed-constant infusion of [1(-13) C] leucine. Plasma volume (PV) using Evans Blue dye dilution and total body water (TBW) using bioelectrical impedance analysis were measured on C, R1, and R8.', 'On R1 as compared with C: 1). PV (+23.3 +/- 3.2%; P<or= 0.001) and TBW (+4.2 +/- 0.8%; P<or= 0.01) expanded; 2). FSR and ASR both increased from 5.36 +/- 0.46 to 6.86 +/- 0.62%.24 h (+ 29.0 +/- 7.1%; P<or= 0.01) and from 103 +/- 10 to 153 +/- 19 mg.kg body weight (-1).24 h (-1) (+47.5 +/- 6.8%; P<or= 0.01); and 3). plasma albumin and protein masses increased (P<or= 0.05) from 1.92 +/- 0.08 to 2.22 +/- 0.14 g.kg body weight (-1) (i.e., +15.3 +/- 4.9%) and from 3.15 +/- 0.18 to 3.56 +/- 0.21 g.kg body weight (-1) (+13.3 +/- 3.9%), respectively. By R8, all these parameters had returned to their control levels except for albumin mass (1.73 +/- 0.08 g.kg body weight(-1), which was lower than C ( P<or= 0.05).'] Is human albumin synthesis increased by an ultra-endurance trial?