Background: The interstitial fluid of the human myocardium was monitored in 13 patients undergoing aortic valve and/or by-pass surgery, before, during and after hypothermic potassium cardioplegia. The regulation of glucose and lactate was studied after sampling with microdialysis. The following questions were addressed: 1) Is the rate of transcapillary diffusion the limiting step for myocardial uptake of glucose before or after cardioplegia? 2) Does cold potassium cardioplegia induce a critical deprivation of glucose and/or accumulation of lactate in the myocardium? Methods: Microdialysis catheters were calibrated in situ with the internal reference technique and interstitial glucose and lactate concentrations were measured before and during cardioplegia and at 25 h and 35 h, postoperatively. Results: The implanted microdialysis probes did not cause any adverse effects. Before cardioplegia, interstitial glucose was ~ 50 % of the plasma level (P<0.001). Interstitial glucose decreased significantly immediately after induction of cardioplegia and remained low (1.25 ± 0.25 mM) throughout cardioplegia. It was restored to pre-cardioplegic level one hour after release of the aortic clamp. Interstitial glucose then decreased again at 25 h and 35 h postoperatively, to the levels observed during cardioplegia. Interstitial lactate decreased immediately after induction of cardioplegia, but returned back to basal level during the clamping period. At 25 h and 35 h after surgery, interstitial lactate was significantly lower than before and during cardioplegia. Conclusions: To our knowledge this is the first study providing data on interstitial glucose and lactate concentrations in the human heart. Glucose transport over the capillary endothelium is considered as a rate-limiting step for its uptake in the working heart, but not during cold potassium cardioplegia despite the glucose deprivation following perfusion of glucose-free cardioplegic solution. Lactate accumulated during cardioplegia but never reached exceedingly high interstitial levels. We conclude that the microdialysis technique provides information, which may be relevant for myocardial protection during open-heart surgery.