The work deals with the characteristics of the Gradole spring in the valley of the Mirna River whose waters have become a part of the water supply. The work consist’s of two parts. The first part deals with the general characteristics of the Gradole spring, its water balance and its behaviour during the last thirty years. In the second part of the work the basic hydrologic analysis of typical data from that source has been carried out. The spring is of exceptional importance for the water supply of the northwest Istria because it is the most abundant karst spring in Istria included in the water supply. During the periods of exceptional drought the water supply system of that area is put under a lot of pressure and it is impossible to meet the demand by supplies from other natural resources in its vicinity. In such circumstances spring’s static water reserves are usually drained and sometimes the amount of the consumption of water intended for water supply is even reduced. The work is an overview of the artificial recharging of the Gradole spring with waters from the Butoniga Accumulation before the construction of the water conditioning facility. The analysis of the hydrologic data resulted in the detection of trends of movements of characteristic flows of the Gradole spring as well as the prediction of the emergence of their extreme values.