Plant secondary compounds are deterrents and toxins to a variety of herbivores. The effect of secondary compounds on water balance of herbivores is virtually unexplored, yet many secondary compounds are renowned for their diuretic effects in humans and laboratory rats. We review data from the ethnopharmocological literature on plants with diuretic effects. We also present our data from experiments on water intake of specialist (Neotoma stephensi) and generalist woodrats (N. albigula) consuming plant secondary compounds from their natural diet. We measured effects of dietary secondary compounds on voluntary water consumption, urine volume and urine osmolarity. Ingestion of secondary compounds increased water intake and urine output and decreased urine osmolarity in both species. However, the generalist was more impacted by dietary secondary compounds than the specialist. Our results combined with that from the literature suggest that diuresis may be a prevalent consequence of ingestion of secondary compounds. Many herbivores live in arid habitats with limited access to free-standing water, thus an increase in the desire for water may have profound consequences on foraging behavior and fitness.