The GASOUT computer code calculates fission gas release, activity release, and fission product swelling in a Canada deuterium uranium (CANDU) fuel element during transient (nonequilibrium) conditions such as load following, postulated accidents involving high temperatures, and temporary postdryout operation of fuel. The phenomena modeled in the code include production of isotopes; diffusion; grain growth, both equiaxed and columnar; sweeping by grain boundaries; growth of grain-boundary bubbles; interlinkage of bubbles; grain-face separation; release by melting; radioactive decay; and effect of precursors. These phenomena are described in the code by rate equations, which are integrated numerically within the code. Therefore, the model is dynamic and provides results during short-term transients (few seconds to few days) as well as at the end of long irradiations (few years). This one-dimensional code was developed for accident conditions that lead to high fuel temperature, but it is also applicable to normal operating conditions. The activity calculations account for contributions from both volatile and nonvolatile fission products. They also account for radioactive decay during all the above processes and for the effect of precursors. The predictions of GASOUT were found to be in reasonable agreement with the steady-state predictions (for stable gas) of the NOTPAT code on which it is based. Furthermore, agreement was also reasonable to exact solutions from the Booth diffusion model, to data from the CONTACT-1 series of experiments and from the direct electric heating experiments, and to American Nuclear Society Standard 5.4.