The effect of the viper Cerastes cerastes cerastes venom and venom fractions on carbohydrate metabolism. Toxicon 31, 791-801, 1993.--I.p. injection of a sublethal dose of Cerastes cerastes cerastes venom into white rats induced a marked hypoglycemia after 15 min. This hypoglycemia continued for at least 24 hr. Hypoglycemia was accompanied by a significant increase of liver and muscle glycogen at 15-30 min and 8 hr. Plasma lactate levels were significantly increased for most of the 24 hr test period, and was accompanied by a marked decrease of liver lactate levels. Levels of skeletal muscle lactate increased significantly. Viper venom significantly increased levels of plasma, liver and skeletal muscle pyruvate for most of the 24 hr test period. Cerastes cerastes cerastes venom was fractionated using gel filtration into six fractions. Each of the first four fractions caused a significant hypoglycemic effect at some point of the 6 hr test period, while fraction I also produced a hyperglycemia 30 min after administration. Fraction III, the only fraction to show a continuous hypoglycemic effect during the 6 hr test period, significantly increased plasma insulin levels 30 min after treatment. It is suggested that the hypoglycemia may be due to a direct effect of venom components on plasma insulin levels.