When a Euglena gracilis in a Ca2+-containing medium is impaled with a microelectrode, the flagellum is instantly ejected. In a Ca2+-free medium to which 1 mM EGTA has been added, the flagellum remains attached to the organism, but it loses activity upon impalement. Externally added ATP at a concentration of 10 mM will sustain normal flagellar activity (at approximately 20 Hz) of an impaled Euglena. If negative direct current of several tenths of a microamp is injected through the impaling microelectrode, the flagellar activity is stopped or much reduced. When the current injection is turned off the flagellum returns to its initial activity. This cycle can be repeated many times on the same animal, independent of whether Mg2+ is present in the external medium or not. If 1 micrometer of gramicidin is added to Ca2+-free medium containing 1 mM EGTA and 10 mM ATP, the flagellar activity becomes dependent on external Mg2+. Without external Mg2+ no flagellar activity is present after one or two current injection cycles as described above. With 1 mM Mg2+ present in the external medium many cycles (up to 10) can be produced. This Mg2+-dependent flagellar activity shows a smooth dependence on the amount of current injected. Observations taken by high speed cinemicrography show that in the third injection cycle the average frequency of the flagellar motion is 16-3 Hz at 0 muA, is 8 Hz at 0-2 muA, and is approximately 0 at 0-6 muA of negative current. The injection of positive current results in an increase in flagellar frequency dependent on the amount of current injected. The data indicate that the control of motility of Euglena flagella is dependent on an electrically activated Mg2+ pump.

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