Hematocrit, the volume fraction of blood composed of erythrocytes, influences the oxygen transport capacity of blood by affecting both the oxygen content and viscosity of the blood. In lizards, variation in locomotor performance also depends on hematocrit. Here, I report that hematocrit, total plasma protein, and consequently blood viscosity increase allometrically with body size in the Western Fence Lizard, Sceloporus occidentalis. Blood viscosity varied exponentially with hematocrit and was substantially higher at 15 C than at 30 C. Hematocrit was lower in hatchling lizards (<five weeks old) than in juveniles and adults and increased significantly between four and five weeks after hatching. The size-dependent variation in these hematological parameters may contribute to size-dependent variation in aerobic performance or may be part of another adaptive ontogenetic change (e.g., in the osmoregulatory system).