Most frogs and toads dehydrate rapidly in terrestrial environments. We designed experiments to assess the impact of such water loss on the locomotor ability of leopard frogs (Rana pipiens), green frogs (Rana clamitans), American toads (Bufo americanus), and marine toads (Bufo marinus). Dehydration of animals to 80% of their standard mass had no effect on the distance covered in three consecutive jumps in the four species. However, loss of body water influenced endurance in specimens of both species of Rana by severely depressing the distance traveled and the time to exhaustion. Moderate dehydration reduced the distance traveled in 5 min by American toads but had no effect on locomotion in marine toads. When the plasma osmolality of leopard frogs was increased by injections of hypertonic saline, sprint performance was not altered, but endurance was reduced significantly. Frogs loaded with isotonic and hypertonic saline solutions had different plasma osmolalities at the beginning of exercise, but exhausted at the same plasma osmolality. Declining performance (fatigue) seems to be related both to initial hydrational status and to the accumulation of metabolites during exercise.