The release of mercury from two freshly triturated amalgams exposed to a reducing atmosphere, hydrogen, was quantitated at three different temperatures. A low-copper and a high-copper amalgam were placed into a flowing hydrogen gas atmosphere for 60 min after trituration, and then the hydrogen was replaced by compressed air. The results were compared to those obtained in a previous study in which air and argon atmospheres were used under identical conditions. At 37 degrees C, the rate of evaporation of mercury from the amalgams was similar when they were exposed to hydrogen before being exposed to air. During exposure to hydrogen, the evaporation rate appeared to exceed the limit of the gold film mercury analyzer (284 pg/mm2/s), but was rapidly reduced upon exposure to air. The results were identical to those from argon exposure. When the same experiment was performed at 80 degrees C, the evaporation rate after hydrogen exposure was greater than that after exposure to argon, and far greater than that during exposure to air alone. Similar results were achieved at 110 degrees C, but there was less difference between hydrogen and argon exposure. The results support previous studies which show that the evaporation of mercury from amalgam is mainly limited by the formation of an oxide film.