Terrestrial crabs confront the problems of salt and water balance and nitrogen excretion in a terrestrial environment with organ systems evolved to function while immersed in an aquatic environment. Their antennal glands produce urine that is essentially isosmotic to the blood, and contains very little nitrogenous waste, yet terrestrial crabs are excellent iono- and osmoregulators in nature. Because voided urine is passed to the branchial chamber for reclamation of ions, the gills, the major organ for ion regulation in aquatic crabs, are still able to perform their ion-regulating role even in air-exposed individuals. The present study demonstrates that, during urine recycling, the gills retain their function as the major avenue for nitrogen excretion as well. In two species of terrestrial crab, Gecarcinus lateralis and Cardisoma guanhumi, the concentration of ammonia increases more than 10-fold in urine during recycling at the gill, greatly enhancing the amount of nitrogen that can be excreted for a given volume of water lost in the urine. Additional results show that without the ability to reclaim ions from the urine by recycling it at the gills, G. lateralis would be unable to remain in osmotic equilibrium.