Accessibility of heart water to pentoses.

1. The raffinose space, the inulin space and the weight loss on compression of perfused rat hearts are indistinguishable in magnitude.2. The inulin space shows systematic variations from day to day and is also affected by the procedure used for removing surplus water from the perfused heart. It is therefore not safe to use a mean figure of extracellular space in permeability experiments.3. Perfusion of hearts with arabinose solutions alters the intracellular water content. These alterations can be accounted for if it is assumed that the arabinose which penetrates the cells is uniformly distributed in the cell water.4. By appropriate choice of pentose concentration and of time of perfusion it can be shown that all the estimated cell water is accessible to pentose.5. It is concluded that no detectable fraction of the extracellular water is inaccessible to inulin and that no detectable fraction of the intracellular water is inaccessible to pentose.