Abstract 1. 1. Metamorphosis of sexually mature Abmystoma larvae is accompanied by a twofold increase in total serum protein. 2. 2. Erythrocytes increase 15–18 per cent during transformation from the larval to the adult state. 3. 3. The average ratio of α-globulin to total globulin for paedogenic larvae is about 2.34 but in intermediates and adults it approximates 0.75. 4. 4. Albumin is lacking in this salamander species but further study may show that the fastest migrating fraction, an α-globulin, has many of the physiology capacities found in albumin. 5. 5. Serum protein changes associated with metamophosis are suggestive of phylogenetic recapitulation. 6. 6. Metamorphic serum protein alterations may serve to increase water regulation in the kidneys and tissues by exerting osmotic influence and promoting blood pressure though plasma volume expansion and increased viscosity.