Methods for the monitoring of glucose and lactate in intensive care units (ICU) based on microdialysis and continuous flow enzyme reactions plus some in vitro and in vivo characteristics of the probes used and the detection systems are described. Two microdialysis techniques were developed for clinical use: in sepsis patients a subcutaneous device for lactate monitoring was placed and in prematurely born infants a transcutaneous device was used for nearly non-invasive sampling of glucose from the skin. There was a relatively strong relationship between transcutaneously sampled and blood glucose in the neonates, on the other hand the relationship between subcutaneously sampled and blood lactate was highly significant but relatively weak. These results and our preliminary results obtained with transcutaneous ethanol monitoring (not presented here) show that in vivo possibilities of our techniques depend on the location of the sampling/detection devices and the chemical nature of the analyte, because these properties determine diffusion characteristics in vivo. The present approach may be an alternative to the use of the more integrated biosensor technology in vivo, since it avoids major problems related to biocompatibility.