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1. Single giant barnacle muscle fibres from Megabalanus psittacus (Darwin) were used to measure the Ca entry and the development of tension in the fibres under membrane potential control.

2. Fibres bathing in 60 mM-MgCl₂ sea water, free of Ca, did not develop tension with sudden displacements of the membrane potential towards more positive values. This failure to develop tension with depolarizations was observed with and without the internal application of Ca buffers.

3. Fibres bathing in artificial sea water with either 10, 20, 60 or 100 mM-CaCl₂ developed tension with depolarization even after 60 min of internal perfusion of the fibres with solution containing no Ca buffers. In this case the maximum tension recorded during a voltage clamp run decreased with time from nearly 2·5 to 0·2 kg/cm². However, addition of 10 mM-Tris-EGTA (ethyleneglycol-bis (-aminoethyl ether) N, N' — tetraacetic acid) to the perfusing solution rapidly eliminated the development of tension; after 10 min of internal perfusion with Ca buffers no tension could be elicited by electrical stimulation.

4. Ca-influx determinations were carried out only in the fibres in which the outward K⁺ currents were blocked by internal application of TEA (tetra-ethylammonium). The ratio of `measured extra Ca influx/computed ionic flux of divalent cations during the inward current' was 1·06 ± 0·41.

5. For fibres bathed in either natural sea water or in artificial sea water with various concentrations of Ca, the temporal course of development of isometric tension was similar to the temporal course of the integral of the inward current due to Ca²⁺.

6. In a fibre from M. psittacus bathing in natural sea water the calculated extra entry of Ca required to increase its internal concentration to about 50 µM was 500 p-mole per depolarization (60 mV); while the corresponding average influx calculated from the inward current record in natural sea water is 474 p-mole.

7. Evidence was obtained for the accumulation of Ca in an internal compartment.

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