The accurate measurement of water intake by rats is essential to many physiological and pharmacological studies. Various methods have been developed to monitor water intake by counting the number of times the tongue touches the drinking spout, e.g. electric contact and photocell type drinkometers. An underlying assumption in these techniques is that each lap results in a constant intake volume. Evidence suggests that this assumption may not hold true. Our method removes the need for this assumption and allows one to continuously monitor actual water volume in the drinking tube. A 52 cm glass tube was equipped with a drinking spout at the lower end while the upper end was sealed with the inlet to a pressure transducer. Since the bottom is open to atmosphere, the pressure in the sealed upper tube where the transducer is located is proportional to water height and volume. The technique was validated by 24 hr monitoring of the water consumption of a rat in a chronic monitoring cage equipped with the drinking tube. To compare behavioral patterns a means for monitoring food consumption and urine output were also included.