From published data for various tissues on intracellular Na, K and Cl were calculated the net anionic charge on all other substances present and also the total contribution of these to osmolality, assuming osmotic equilibrium with extracellular fluid. These parameters were compared for muscle and nerve of animals differing widely in osmolality and also for other mammalian cells. Cell volume regulation in some euryhaline species was considered; it is only partly due to ninhydrin-positive materials. In sheep mammary glands K seems to be sequestered with lactose and anion. Changes in mammalian muscle due to adrenalectomy, hypophysectomy, K deficiency, myotonia, acid-base imbalance and treatment with deoxycorticosterone or insulin were discussed.