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1. The uptake of ²⁸Mg, and the total tissue content of Mg, Ca, Na and K have been determined in the smooth muscle of the guinea-pig taenia coli. The Mg content was 6·56 m-mole/kg fresh wt. immediately after dissection, falling slowly to 5·11 after 6 hr immersion in Krebs solution at 37° C.

2. The Mg content rose to 15·4 m-mole/kg fresh wt. during immersion in isotonic sucrose containing only MgCl₂. It was independent of the Mg²⁺ concentration in this solution, but was depressed when K⁺ or Ca²⁺ ions were added.

3. ²⁸Mg uptake showed three separate phases, extracellular, intermediate and slow. The size of the extracellular phase was proportional to the Mg²⁺ concentration in the solution, but the size of the slow phase was constant. The size of the intermediate phase, exchanging with a half-time of a few minutes, was depressed when K⁺ or Ca²⁺ ions were added.

4. The results are compatible with a competition between Mg in the intermediate phase of tracer exchange, and K⁺ or Ca²⁺ ions for fixed anionic sites in the tissue.