We analyze the diffusion of O 2, CO 2 and non-volatile products in tissue cultures on various gas-permeable and impermeable supports. From the equations presented, it is possible to predict the steady state pericellular P o 2 and P co 2 as well as the accumulation of non-volatile products by cells in culture. In contrast to non-permeable supports, cultures with high respiration rates on highly permeable supports will not deplete their pericellular P o 2 or accumulate high pericellular P co 2 which lowers pericellular pH. Due to a higher steady state P o 2, dependence on glycolysis and the resulting drop in pericellular pH will not occur on highly permeable supports. We consider various problems in cell culturing in the light of our calculations and conclude that the pericellular environment of cultures on highly permeable supports can be accurately controlled by control of the incubator atmosphere.