1. A sudden and maintained pressure decrease from a value of one atmosphere to points between 625 and 300 mm. Hg results in an increase in gas content of guppy swimbladders and the rate of increase is an inverse function of the pressure. 2. This increase in gas content has been shown to be the result of lowering the swimbladder gas pressure, the gas tension of the body fluids then favoring passage of gases into the bladder. 3. Guppies are able to oppose more or less successfully the inward diffusion of gases at first, but gradually in the course of two to five hours such opposition ceases. 4. The transitory attempt on the part of the fish at density adjustment in response to the pressure decreases is explainable in the presence of an initially high partial pressure of CO2 in the swimbladder. 5. The guppy appears to be unable to remove gases from the bladder when the diffusion gradient does not favor such passage.