We describe a pressure chamber designed for measuring fluxes and equilibrium distributions of substances between cells in suspension and the extracellular medium at pressures up to 400 ATA. The pressure chamber contains a filtration apparatus capable of taking four separate samples of cell-free medium from a cell suspension at timed intervals, without necessity of decompression. In addition, a high pressure injection system makes possible the addition to a cell suspension of precise quantities of agents such as ionophores or metabolic inhibitors, for observations of their effects on cell functions at pressure. The chamber of filtration apparatus functioned successfully in preliminary experiments designed to measure the equilibrium distribution of 36Cl- in human erythrocytes at high pressure.