Swelling and rare gas release of irradiated prototype EBR-II fuel pins during heating have been studied with material containing 320 to 830 ppm of these gases (0.2 to 0.6 at. % burnup). Data on both phenomena were obtained at varied heating rates and with stepwise heating to successively higher temperature levels. In each experiment, over 99% of the rare gases was released before the alloy was fully molten at about 1080°C. Within experimental error the behaviors of xenon-133 and krypton-85 were identical. Below 750°C, gas release was slow; above 750°C, the rate increased sharply. The quantity of gas evolved as a function of time at constant temperatures to 850°C appeared to be directly proportional to the time rather than the square root of time as predicted by diffusion theory. Swelling of the alloy showed much the same type of temperature dependence as the release of rare gas. For all heating patterns, pin swelling was most pronounced above 750°C, reaching a maximum diametral increase of about 47% as the pin melted. A brief comparison between the release of rare gas from this alloy and that from other metallic fuels is made. The effects of this phenomenon and associated swelling on the melt refining process envisioned for recovery of fuel from the first core loading of EBR-II are discussed.