1. In Gammarus pulex and G. duebeni sodium influx from sodium chloride solutions and sodium loss into de-ionized water were not affected in a consistent manner by the presence of calcium ions at concentrations up to about 2 mM/l.
2. Sodium influx in G. pulex was depressed by the addition of 2-10 mM/l. calcium.
3. Sodium influx in G. zaddachi was stimulated by the addition Of 2-4 mM/l. calcium chloride but depressed by the addition of 20-53 mM/l. calcium chloride.