The distribution of the Na-K-Ca index values in the geothermal aquifer of Cerro Prieto indicates a probable cold-water recharge in the northern and western parts of the present producing field. In the central part of the field, the distribution of Na-K-Ca values is very irregular, due to the effect of percolation or the vertical descending flow of colder waters caused by apparent over-exploitation. The large area with indices varying between 0·5 and 0·7 to the east, southeast, and south of the field confirm the presence of hotter geothermal waters than those observed in the present producing field. The limits of this hot aquifer have not yet been determined toward the east and southeast. The temperature distribution in the Cerro Prieto aquifer confirms the conclusions reached using the Na-K-Ca index. The maximum temperature calculated was in well M-53 (350°C) and the minimum was in M-9 (250°C), when wells M-6 and M-1A, which have lower temperatures, are not taken into account. The chloride distribution in the aquifer confirms the recharge of colder, less saline waters in the northern and western parts of the field. It was surprising to find that the chloride content in the aquifer was lower in the eastern part (M-53) than in the present field, even though the temperature is higher. Another interesting fact is the extremely low chloride content found in well M-101, with temperatures around 290°C. The distribution of potassium and silica in the aquifer was not as useful for the interpretation of fluid movement as originally expected. Discrepancies were found with respect to the interpretations based on the Na - K - Ca index and the chloride content. In regard to the changes in the Na - K - Ca index, in temperature, and in chloride content during the exploitation of the field, a gradual increase in Na - K - Ca index values was observed in wells with high and low enthalpy. In exceptional cases these values decrease or remain constant. This increase in the index has represented a 10 to 20°C temperature decrease with respect to the original values existing at the beginning of the exploitation of the field (1973). The chloride content in the aquifer has decreased at different rates, from 670 mg/l per year in M-26 to 371 mg/1 per year in well M-42, even though in some wells (M-20) it has increased and in others (M-5) it has remained almost constant. In order to decrease the rate of temperature and pressure reduction in the present producing field, it is recommended that production of low enthalpy wells be suspended and be replaced by new wells located in the periphery of the field, preferably to the southwest and northeast. Thus, the production area would at least be doubled, in an attempt to avoid the percolation or vertical flow, which is apparently causing the cooling of the reservoir. Another measure, which could help prevent a rapid drawdown, is to exploit the deeper hot aquifers that apparently exist in the center of the field. This would first have to be confirmed by an exploratory well or by deepening one of the present wells.