Abstract: An automatic control system for maintaining the quality of water in a cooling tower utilizes a probe which senses the oxygen reduction potential (ORP) of the water for activating a chemical supply means, in accordance with a pre-established ORP measurement, to maintain the ORP of the cooling tower water between lower and upper limits. A second sensing probe measures the conductivity (in MHos) of the water as a factor of the total dissolved solids (TDS) to control the bleed-off or blow-down of the water. The chemicals, which are supplied for maintaining the ORP, permit significantly higher total dissolved solids in the water than with standard cooling tower systems, without the buildup of scale. As a consequence, water consumption is significantly reduced; and the system functions automatically, without requiring periodic visual inspection, water analysis or manual operation.