We tested the role of increased ammonia in urine as an energy-and/or nitrogen (N)-saving mechanism in the great fruit-eating bat Artibeus lituratus (Phyllostomidae). We compared N excretion in two groups of bats fed energy-rich (2.75 kJ g -1 wet mass) or energy-poor diets (0.7kJg -1 wet mass). Within each diet, bats were assigned to different N contents. In order to function as an energy-saving mechanism, ammonia production should increase with decreasing energy intake. To function as an N-saving mechanism, ammonia production should increase with decreasing N intake. Because we varied both diet energy density and N content, our study design allowed us to test these two possibilities simultaneously. Bats had higher food intake rate and, consequently, higher N intake rate on the energy-poor diet, but energy intake rate was lower. Most bats on the energy-rich diet were ureotelic whereas on the energy-poor diet bats were ureotelic, ammonotelic or ureo-ammonotelic. Bats fed the energy-poor diet had a higher excretion rate of ammonia and a higher percent of N excreted as ammonia. Percent N ammonia and ammonia excretion rate were inversely related to energy intake, but they were not related to N intake. By favoring ammonia production over urea, bats on the energy-poor diet may save up to 1% of their basal metabolic rate. Consumption of energy-dilute fruits by fruit bats might affect the way in which N wastes are excreted, favoring the excretion of ammonia N when food intake is accompanied by the ingestion of large volumes of water. © 2011. Published by The Company of Biologists Ltd.