A 500 cc. initial vol. N2 pneumoperitoneum in the cat diminishes at a rate of 30 cc./day for the first 8 days and completely disappears in 23 days. If the initial gas is SFg, the initial volume doubles by 9 days due to more rapid influx of N2 than efflux of SFg due to the low solubility of the latter. Beyond the point of max. vol. the total vol. diminishes slowly so that the initial vol. is not reattained for 30 days. The dynamics of these gas transfers are discussed.