1. The muscle of the isolated fore-gut of Dytiscus marginalis will maintain rhythmic contractions for long periods when suspended in a medium of the composition NaCl 0.161 M : KCl 0.003 M : CaCl2 0.002 M at a pH of about 7.2.
2. In the presence of normal concentrations of sodium, alterations in the potassium and calcium concentrations cause changes in the muscle essentially similar to those already described by Hogben and by Wells in the crustacean heart.
3. Rhythmic contractions can be maintained for extended periods in mixtures of potassium and calcium chlorides alone.
4. In potassium-calcium mixtures the tone is always high, but the contractions are normal in form and the muscle reacts in the normal manner to changes in the K/Ca ratio. The optimum K/Ca ratio is extraordinarily low, having a value of about 0.4.
5. The tone of the muscle is dependent, not only on the absolute concentration of potassium and calcium but also on the K/Ca ratio.