Air-breathing Cardisoma carnifex, collected in Moorea, French Polynesia, were held in fresh water similar in chemical composition to that in their burrows. Under control conditions, which allowed branchial chamber flushing but not ventilation of the medium, crabs demonstrated net Na+ and Cl− uptake, and ammonia, urea and base excretion (= acidic equivalent uptake). Throughout 192 h of water deprivation, crabs dehydrated slowly at a rate of 0.55 g H2O kg−1 h−1, eventually reaching a near lethal 18% loss of total body water. Increases in haemolymph osmolytes were quite variable (0–29%); electrolyte excretion was negligible. MOO2 and MCOCO2 both decreased by approximately 55%, maintaining an unusually low gas exchange ratio (R = 0.53), and suggesting general metabolic depression. There was no evidence of internal hypoxia as haemolymph lactate remained at hydrated levels and PaOO2 actually increased. The dominant acid-base response was a progressive metabolic alkalosis accompanied by a partially compensating rise in PaCOCO2. Alkalosis was probably caused by blockage of the normal aquatic excretion of base produced by the metabolism of this herbivore. Other possible causes were eliminated: i.e. alkalaemia due to contraction of the ECFV; entrainment via strong ion shifts; CaCO3 mobilization; and ammonia accumulation in the haemolymph. In the absence of water, net ammonia production and excretion both appeared to cease, and alternate end products (urea, uric acid) did not generally accumulate. Within 2h of rehydration, crabs regained more than half the lost water, MOO2 and MCOCO2 increased above control levels, and ammonia excretion and haemolymph concentration both exhibited a prolonged (56 h) 4- to 6-fold rise. At the same time, metabolic alkalosis was reversed in association with elevated net base excretion into the water; the latter was correlated with an increase in the strong ion difference (SID) flux ([Na+ + K+ + Ca2+ + Mg2+ - Cl−]).