Ad lib water intake of rats displays two prominent features: Water intake is temporally and quantitatively associated with food intake, and water intake is nocturnal. Physiological mechanisms linking water and food intake suggest that the rhythmicity in water intake may be at least partially driven by a rhythmicity in food intake. The present study examined whether the daily rhythm of rats modulates the association of food and water intake. Rats were housed under light/dark cycles and continuously monitored for temporal and quantitative patterns of ad lib food and water intake. The results indicate that the water/food ratio is much lower for meals consumed in the light phase than in the dark phase. The above results suggest a rhythmic modulation of the mechanisms linking food and water intake. We suggest that much of the complex rhythm of drinking stems from several interacting factors. 1) Food intake is nocturnal. 2) Food intake induces water intake by physiological mechanisms. 3) The amount of water intake induced by food intake is influenced by rhythmic factors.