Application of beta-adrenoreceptor blockers in asthmatic patients increases broncho-constriction, and angiotensin converting enzyme inhibitors may induce cough. In either case life quality of asthmatics may deteriorate. Increased sympathetic activity plays a significant role in the development and maintenance of arterial hypertension. Recently imidazoline receptor agonists have been used in the treatment of hypertension. The action of these drugs results in modification of the vegetative nervous system due to a decreased activity of the adrenergic system. Such modification could be disadvantageous in asthmatic patients in whom an increased parasympathetic tonus is an important mechanism of bronchoconstriction. The aim of the paper was to assess the autonomic nervous system during spontaneous overnight sleep in 10 mild or moderate asthmatic women in a stable stage of the disease; they were all treated for mild or moderate arterial hypertension with Tenaxum 1 mg daily. The examination consisted in spectral analysis of the heart rate variability of 10 min. periods repeated every five minutes. The following parameters of the spectral power were studied: high frequency components (HF) reflecting parasympathetic activity, low frequency components (LF) representing sympathetic activity, as well as the LF/HF ratio. The investigations, including spirometry, were performed before and on the 3rd and 21st day of treatment. Arterial tension was brought to normal in 7 out of 10 patients. Spirometric parameters (VC, FEV1, MEF50%FVC, MEF25%FVC) did not undergo any significant changes, however a significantly increased parasympathetic activity (HF) in the studied patients was found on day 21. The results of our investigations indicate that Tenaxum can be a safe therapeutic agent in the treatment of mild or moderate arterial hypertension in asthmatics.