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 131I-labelled ovine gamma globulin. Following injection of 82Br (100 micronCi), 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. 131I-labelled ovine gamma globulin (20 micronCi) was injected after the 3-h 82 Br space was obtained and blood samples were taken at 10, 20, 30 and 40 min. In 16 determinations in 11 sheep (25-47 kg body weight) the mean (+/- s.e.m.) ECFV was 9112 +/- 289 ml (or 245 +/- 9 ml/kg). The mean PV for 16 observations in 11 sheep measured together with ECFV was 1597 +/- 62 ml (or 42-8 +/- 1-8 ml/kg). Although there was no relationship between body weight and PV there was a significant correlation between ECFV and body weight and also significant negative correlations between body weight and ECFV or PV when these were expressed as a function of body weight. The variation in ECFV measured on four occasions over 7-10 days in four sheep was 3-5% (range 2-6-4-6%). For PV measured in two animals on two consecutive days at the same time as ECFV the coefficient of variation was 1-5 and 2-1%. Acute sodium depletion (250-670 mmol) by parotid duct cannulation in three sheep resulted in a fall in ECFV which would account for only 15-20% of the sodium deficit. The remainder is presumably derived from ruminal sodium stores.