The diurnal wind system of the Kali Gandaki valley has been explored in September and October 1998 in a field campaign using pilot balloons as the main observational tool. This valley connects the Plateau of Tibet with the Indian plains. The river crosses the Himalayas forming the deepest valley on earth. It is between this gorge and the Plateau of Tibet that intense upvalley winds are blowing during the day. Observations have been made along the river at various spots selected between the exit point from the Himalayas and the source close to the Plateau of Tibet. It is found that the upvalley winds are strongest between Marpha and Chuksang (Fig. 1). The upvalley wind sets in first at the ground but a layer of 1000-2000 m depth forms quickly after the onset. This deep inflow layer persists up to Lo Manthang, a city located a few kilometers south of the Plateau of Tibet. Deceleration in the late afternoon and evening commences also near the ground. Weak katabatic winds have been observed to form late in the night. Causes of these unique phenomena are discussed.