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Plant secondary compounds are recognized deterrents and toxins to a variety of herbivores. The effect of secondary compounds on water balance of herbivores is virtually unexplored, yet secondary compounds could potentially cause a decrease in an animal’s ability to maintain water balance. We investigated the effects of secondary compounds, alpha-pinene and creosote resin, on water balance in three species of herbivorous woodrats (Neotoma stephensi, N. albigula, N. lepida). In separate experiments, we measured the effect of these secondary compounds on voluntary water consumption, urine volume and urine osmolarity. In both experiments, water intake and urine volume increased and urine osmolarity decreased compared to controls. Water balance of specialist or experienced woodrats was less affected than generalists and woodrats with less prior experience with particular secondary compounds. Our results suggest that secondary compounds have diuretic-like effects on herbivores. Woodrats live in arid habitats with limited access to freestanding water; thus an increase in water requirements may have profound consequences on foraging behavior and fitness.