Methods are described for the simultaneous measurement of extracellular fluid volume (ECFV) and plasma volume (PV) in sheep using dilution of 82Br (as sodium bromide) and l3lI-labelled ovine gamma globulin. Following injection of 82Br (100 }lCi), equilibrium in blood was reached after 3 h at which time only 4 % of the injected dose was in rumen water. The ECFV was measured as the mean Of the 2- and 3-h bromide space after correction for the relative water content of plasma, the Gibbs-Donnan factor and the loss of 82Br into red blood cells. l 3lI-labelled ovine gamma globulin (20 }lCi) was injected after the 3-h 82Br space was obtained and blood samples were taken at 10, 20, 30 and 40 min.