Methods for the continuous in vivo monitoring of glucose and lactate were applied in man for studies of peripheral metabolism and, in rats, for cerebral examinations. Lactate ('lactography') and glucose were continuously measured by the enzyme formation of NADH or NADPH using appropriate dehydrogenases. Glucose was monitored with a transcutaneous probe in neonates or with probes placed directly in the brain of conscious rats. In neonates a high correlation was seen between the dialysate and blood levels. A correlation between the age of the neonates and glucose levels in the dialysate was found, suggesting that the thickness of the skin determined to a large extent the diffusion of glucose into the probe. In the rat brain transient changes in the extracellular glucose levels were seen during electroconvulsive shock and immobilization stress. Lactography was applied to striatum or hippocampus during stress, electroconvulsive shock or systemic and intracerebral drug applications. Extracellular lactate decreased during intracerebral deoxyglucose infusion and was increased by excitatory neuronal activity and the presence of glucose in the perfusion medium. The present studies show that microdialysis in combination with continuous flow analysis can be used to study metabolic trafficking in vivo and in clinical studies. In the brain both neurons and glia may contribute to extracellular lactate levels.