Magnesium/chlorine and sulphur/chlorine ratios have been measured in the body fluids of some planktonic organisms by x-ray emission analysis of frozen hydrated specimens in a scanning electron microscope. Homarus vulgaris (Anthropoda: Decapoda) larVae excluded Mg2+ and SO42- ions from the haemolymph, but to a lesser extent than does the adult lobster. Bipinnaria larvae of Asterias (Echinodermata) excluded Mg2+ and SO42- ions from the coelomic fluid. Obelia medusae excluded Mg2+ and SO42- ions from the mesogloea but concentrate them in the gastrovascular system. The high concentration of sulphate in the gastrovascular fluid of medusae has been confirmed by rhodizonate titration in Cyanea and Rhizostoma jellyfish. Some implications of magnesium and sulphate regulation are discussed.