Abstract
Lowering the potassium in the sea water from 0.011 M to 0.006 M caused an exit of potassium from cells of Valonia macrophysa. Sodium continued to penetrate and the ratio K / Na fell off. The cells ceased to grow but there was no evidence of injury. Increasing the external potassium brought about an increase of the internal concentration of potassium, of halide, of total cations, and of the ratio K / Na inside. These phenomena are to be expected on theoretical grounds.