Abstract
1. 1. The dependence of contractile vacuole activity upon the levels of cytoplasmic Na+ were studied in Paramecium aurelia by means of drugs capable of increasing or decreasing membrane permeability to Na+. 2. 2. The Na+ ionophore monensin produced an abrupt and reversible decrease in the frequency of contraction. This effect was partly dependent upon the presence of Na+ in the medium. 3. 3. The diuretic amiloride, known to hinder Na+ influx in many cells, increased vacuole activity, although it did not prevent a decline in frequency induced by a sudden rise in the transmembrane gradient of Na+. 4. 4. The results suggest an inverse relationship between Na+ levels in the cytoplasm and the frequency of contractile vacuole function.
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