A level of myocardial norepinephrine (NE) is considered as one of the meaningful parameters in the estimation of myocardium functioning. Long lasting changes of myocardial NE appear to be not only a sequence of pathologic processes in myocardium, but could also be a factor responsible for some diseases. An improved microdialysis sampling technique with HPLC-ED analysis was developed to measure in vivo NE content in a rat myocardial interstitium. Linear polyacrylonitryl 5 mm probes were flushed with methanol while being immersed in water. In vitro NE recovery with the flow rate of perfusate was found at the level 67.9 + 0.4%. Probes were implanted into the rat myocardium under the general anaesthesia. Myocardial dialysate was collected during 20 min into plastic vials, containing 10 microliters of 0.1 M perchloric acid. Low noise, high sensitivity dual electrochemical detection was used for the determination of NE amount in the samples. Overall sensitivity threshold was found about 0.3 pg of NE in 20 microliters. Steady state concentration of NE in myocardial dialysate was found 54 + 7 pg/ml. 20 min before cardiac arrest the concentration of NE in myocardial dialysate was found to be increased 4-5 times. 40 min after cardiac arrest further increase in NE content was registered up to 1-4 ng per ml of perfusate. It is suggested that the method will be useful for routine study of myocardial NE.