Three synthetic cariogenic diets containing either 25, 10 or 5% of casein with 52% sucrose were given to 3-week-old rats for either 1, 5, or 11 weeks. Stephan 580 cariogenic diet was also tested. Estimations were made of the haemoglobin concentration, packed cell volume, mean corpuscular haemoglobin concentration, and the erythrocyte count. Serum levels were determined for the total plasma protein, albumin, globulin, alkaline phosphatase and vitamin A. Anaemia and lowering of serum albumin, alkaline phosphatase and vitamin A occurred in the animals on low protein diets. Similar trends in haematological values were seen at the fast growing phase in rats on diets of ‘borderline adequacy’. Since it is possible that such changes in the blood may influence the formation and maturation of enamel in the rat, it was concluded that side effects should be prevented by ensuring that high sucrose cariogenic diets have an adequate protein-calorie ra