Abstract 1. 1. Increased levels in serum concentrations of calcium, magnesium, inorganic and total phosphate and total protein were detected before moult, and decreased levels thereafter. 2. 2. Measurements of nitrogen, magnesium, calcium and phosphate content of the hepatopancreas indicate storage during intermoult of material necessary for the synthesis of the new exoskeleton. 3. 3. Urinary and serum calcium concentrations were compared during the moult cycle. Urinary excretion appears to reduce the high serum level before moult. 4. 4. Serum sodium and chloride concentrations vary little throughout the moult cycle, but serum potassium decreases markedly before moulting.