The effects of fasting and increased feeding on plasma thyroid hormones, glucose, and total protein levels were investigated in immature greens (Chelonia mydas) and Kemp's ridley (Lepidochelys kempi) sea turtles. Under constant temperature, nutritional status affected plasma thyroid hormones and glucose level, but not protein level in green and Kemp's ridley turtles. Plasma T₄ in green turtles decreased with deprivation of food, but did not do so in Kemp's ridleys. Increased feeding did not affect circulating levels of plasma T₄ in either species. Plasma T₃, in contrast, tended to decrease during fasting and increase with refeeding or increased feeding. Blood glucose level fell with deprivation of food, but remained constant during increased feeding. Sea turtles may use glycogen or lipid during short-term fasting as in endothermic vertebrates.