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A correlation study has been carried out on data obtained by Kraupp et al. [1] for the myocardial levels of citrate, malate, 2-oxoglutarate, acetoacetate, 3-hydroxybutyrate, intracellular glucose (estimated), glucose 6-phosphate, fructose 6-phosphate, fructose 1,6-diphosphate, pyruvate, lactate, glycogen, ATP and free fatty acids (FFA) in relation to the corresponding blood glucose level in I-normal, II-acute alloxan-diabetic, and III-chronic alloxan-diabetic rats in vivo. A variation in blood glucose level within each group had been achieved by starvation of about 50 % of the animals. In the normal rats there was a significant, negative correlation between the blood glucose level and the myocardial levels of citrate, malate and 2-oxoglutarate, acetoacetate and 3-hydroxybutyrate, and glycogen. A significant, positive correlation was obtained between the blood glucose level and the estimated intracellular glucose level and the fructose 1,6-diphosphate/ fructose 6-phosphate ratio in the myocardium. In the diabetic rats, a generally reverse pattern was obtained. The acute alloxan-diabetic rats showed a significant, positive correlation of the blood glucose level to the myocardial levels of citrate and malate (not 2-oxoglutarate), acetoacetate and 3-hydroxybutyrate, estimated intracellular glucose, hexose monophosphates, lactate, and the lactate/pyruvate ratio. The fructose 1,6-diphosphate/fructose 6-phosphate ratio was negatively correlated with the blood glucose level. In the chronic alloxan-diabetic rats myocardial citrate, malate, estimated intracellular glucose, glucose 6-phosphate and pyruvate were positively correlated with the blood glucose level. The negative correlation of the fructose 1,6-diphosphate/fructose 6-phosphate ratio to the blood glucose level became of even greater significance than in group II (acute diabetic) animals and a negative correlation in respect to the myocardial ATP level appeared. The myocardial ketone-body levels were uniformly low and showed no correlation to the blood glucose level. In no group was there any correlation between cardiac FFA levels and the blood glucose level. In another series of experiments with fed and starved, acute alloxan-diabetic rats, there was no correlation between the blood glucose level and the extracellular levels of FFA, ketone bodies, lactate or pyruvate, nor the 3-hydroxybutyrate/aeetoacetate and lactate/pyruvate ratios. A working theory is proposed for the correlation between certain myocardial substrate levels and the blood glucose level in diabetic rats. [ABSTRACT FROM AUTHOR]