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1. A new perfused preparation of frog's ventricle is described, whose main advantage is that there are short diffusion distances between the cells and the washing fluid.

2. This preparation responds within a few seconds to alterations in sodium, potassium or calcium concentration, or alteration in the osmotic pressure of the bathing fluid. The speed of these alterations is consistent with the diffusional distances involved measured histologically.

3. On depolarizing the preparation with Ringer + excess potassium, tension development starts at about -25 mV and is still increasing at -8 mV (the lowest voltage studied). As in earlier experimetns in frog ventricle (Niedergerke, 1956b) and in skeletal muscle (Hodgkin & Horowicz, 1960) the relation between potential and tension is very steep.

4. Sodium free contractures (Lüttgau & Niedergerke, 1958) are very rapid in onset with this preparation. To maintain the tension developed, a rapid rate of washing is necessary for 1-2 min. After this, slow washing is sufficient. It is proposed that this is due to the leaching out of cellular sodium.

5. In fresh preparations the twitch and maximum K contracture are of similar size when stimulated at 30/min in 1-2 mM-Ca. With the onset of hypodynamia the twitch tension falls, but the K contracture remains unaltered. In hypodynamic ventricles the sensitivity of the twitch to the ratio [Ca]/[Na]² declines, whereas that of the K contractures remains unaltered. Hypodynamia therefore does not affect the contractile elements themselves.

6. During the staircase phenomenon the K contracture and twitch size alter in a similar manner, as previously described by Niedergerke. The sodium free contracture, however, remains unaltered. This tends to favour the hypothesis that the staircase phenomenon is due to effects at the cell membrane rather than in the cell interior.

7. Maximum K contractures occur at a [Ca]/[Na]² ratio of about 0·7 x 10^-4 mM^-1, a figure similar to that obtained in earlier experiments (Lüttgau & Niedergerke, 1958).

8. The length—tension curves of K contractures are similar to those previously described for heart muscle, using single twitches to generate tension.

9. Application of K free solutions produced no rapid contractures in this preparation.