1. In adipose tissue and in skeletal muscle the extracellular noradrenaline levels were studied by microdialysis in the conscious dog and compared with the noradrenaline concentration in arterial plasma.
2. The experiments were performed with and without tyramine added to the perfusion medium, and noradrenaline was measured by a sensitive radioenzymic assay.
3. In the absence of tyramine, the interstitial noradrenaline levels in adipose tissue and skeletal muscles were similar to arterial blood concentrations, provided that the former were corrected for recovery. The recovery estimated from experiments in vitro averaged 16% at room temperature.
4. With tyramine added to the perfusates, noradrenaline levels increased 10-fold. Arterial noradrenaline concentrations did not change, indicating that noradrenaline was released only locally in the tissue.
5. Our results indicate that the microdialysis technique combined with a sensitive assay for measuring noradrenaline may be applicable to the assessment of local noradrenaline release in adipose tissue and in skeletal muscle. This may be of interest, especially in adipose tissue during physiological stimulation in which sympathetic activity is difficult to evaluate by other techniques.