1. Cilia of Paramecium candatum extracted with Triton X-100 were reactivated in the presence of ATP and Mg2+.

2. The beating frequency of the reactivated cilia is a function of both the ATP and Mg2+ concentrations.

3. The reactivated cell models swam forward when the Ca2+ concentrations in the ATP-Mg2+ medium was kept below 10-7M. They swam backward when the Ca2+ concentration was above 10-6M. This was due to a reversed orientation of the effective power stroke of the reactivated cilia.

4. In the absence of Mg2+ the cilia failed to beat, even though ATP was present. If Ca2+ was then added the cilia assumed a new orientation, pointing toward the anterior without beating.

5. Ni2+ inhibited ciliary beating in the reactivated models, but has no influence on changes in the orientation of the cilia produced by ATP and Ca2+. This suggests that one ATP-activated system is responsible for beating, while another governs the direction of the effective stroke.

6. Mn2+ is half as effective as Mg2+ for inducing ciliary beating in the extracted models in the presence of ATP.

7. Salyrgan strongly inhibits Mg2+-ATP activated ciliary activity of the model.

8. Bioelectric control of ciliary activity by the cell membrane of live animals is discussed.

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