1. This review summarizes current knowledge relating to the volume-regulatory and osmoprotective functions of amino acids in mammalian cells exposed to anisosmotic fluids. 2. Experiments in vivo and in vitro have established that they play a significant role in regulating brain cell volume under these conditions, and that taurine may be of particular importance in this respect. 3. Their possible role in renal medulla is discussed, and it is suggested that they may protect cells against acute (but not long-term) osmotic variation. 4. Evidence is briefly presented regarding adaptive changes in amino acid content of other cell types.