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In this study we characterized acid, ammonium and total urate excretion in the faecal pellets of unfed locusts (Schistocerca gregaria) and examined the effect of haemolymph acidosis (HCl injections into the haemocoel) on net acid and nitrogen excretion. In unfed, uninjected locusts, the pH of the urinary pellets was less than 5, and ammonium was excreted at three times the rate of total urate. Ammonium was present primarily as a precipitate, indicating that ammonium excretion is compatible with water conservation in this desert locust. Ammonium excretion was increased by HCl injections, theoretically accounting for 15% of the acid equivalents removed from the haemolymph during recovery from acute acid loads. Luminal pH in the hindgut was affected by feeding state but not by haemolymph acidosis. HCl injections did not affect faecal pellet pH or the excretion rates of bicarbonate, titratable acid, pellet buffer compounds (urate, inorganic phosphate), Na+, K+, Ca2+, Mg2+ or Cl-. In unfed locusts, the low rate of excretion and low pH of faecal pellets may limit the capacity to increase titratable acid excretion after acid-loading.