1. Plasma volume was measured by T-1824 dye dilution and extracellular (ECF) fluid and total body water (TBW) by 82Br and 3H 2O dilution in fresh (FW) and salt water (SW) acclimated Peking ducks, Anas platyrhynchos, and fresh water acclimated gulls, Larus glaucescens, and White Leghorn roosters, Gallus domesticus, before and after intravenous injection of hypertonic NaCl. 2. Plasma volume (%BW) was least in the rooster, 4·4%, and greatest in the gull, 7·2%, but these relationships did not hold when data was expressed as % TBW. When referred to TBW, the rooster remained lowest, 7·8%, but the ducks were highest, FW, 9·2% and SW, 10·6% and the gulls were 8·1%. 3. ECF, calculated as % BW, was essentially the same in FW ducks, 24·9%; SW ducks, 26·4% and roosters, 28·8%, but higher in gulls, 38·2%. When calculated as % TBW FW ducks were 36·4%; SW ducks were 41·3% and gull, 43·6% while the rooster was 52·9%. 4. TBW calculated as % BW was least in the rooster, 54·3%; greatest in the gull, 87·9% and intermediate in the ducks, FW duck, 68·5% and SW duck 64·0% respectively. 5. Injection of 0·2% body weight 5·0 N NaCl caused increases of more than 20% in the extracellular sodium concentration in all birds, however, secretion from the salt glands was observed only in birds which also had a significant increase in ECF volume implicating volume receptors in the stimulatory sequence leading to salt gland secretion.