Investigations have been conducted with a green alga, Scenedesmus acuminatus, into the relationship between the amount and rate of copper accumulation, reduction in growth and changes in K+, Mg++, Na+ and Ca++ during copper uptake. In light, aerated conditions the presence of calcium (10 μM) in the treatment medium decreased the rate of copper uptake and exchange of potassium, magnesium and sodium, and increased the concentration of intracellular copper required to inhibit growth. In dark, anoxic conditions calcium very much enhanced the recovery of copper treated algae despite the accumulation of large amounts of intracellular copper. It is suggested that the influx of calcium during copper treatment protects sensitive ligands. The changes in K+, Mg++ and Na+ content of the cell during treatment may also have a role in the onset of growth inhibition.