Despite evidence of hypervolemia following endurance training, there is little information regarding corresponding extravascular fluid volumes. Quantification of such volumes relies upon radionuclide dilution methods, previously hampered by the loss of plasma albumin. It was our purpose to measure human body-fluid distribution in eight endurance-trained males, using a simultaneous radionuclide dilution technique, incorporating radioiodinated serum fibronogen (RISF). Fluid distribution was measured on three occasions, using 2 μCi of RISF, 8 μCi of 51Cr-labeled erythrocytes, and 20μCi of Na82Br and 450 μCi of 3H2O; to measure PV, erythrocyte (RCV), extracellular (ECFV), and total-body water (TBW) volumes, respectively. Respective volume means, standard deviations, and coefficients of variation were: 46.6 (±4.9; 8.44%), 33.3 (±2.9; 3.89%), 258.1(±12.1; 4.93%), and 654.2 (±13.4; 3.24%) ml · kg-1. The incorporation of RISF provided a reliable modification to previous methods, and revealed a body-fluid expansion in endurance-trained males. It was concluded that such subjects were hyperhydrated, possessing proportionately expanded fluid volumes throughout both intravascular and extravascular spaces. This was attributed to training history and accompanying reductions in adiposity.