In this paper, an attempt is made to estimate the formation environment of the analcime alteration zone in Miocene tuffs around Kuroko(Black ore)-type mineral deposits. Natural mordenite tuff and mordenite-opal tuff of Miocene age were subjected to alteration in sodium carbonate solution (1 molar; 0.25 molar) at 100 degrees to 300 degrees C and 30 to 110 kg/cm 2 for 2 to 20 days using a test-tube-type reactor. When these were treated at 150 degrees to 200 degrees C, analcime (NaAlSi 2 O 6 .H 2 O) was formed as an alteration product. At 300 degrees C, most of the analcime was decomposed with the elapse of time. In the dilute solution (0.25 molar) at 300 degrees C, analcime was secondarily altered to albite. Albite formed more readily in the mordenite-opal tuff (high-silica tuff) than in the mordenite tuff.On the other hand, mordenite and mordenite-opal tuffs showed no remarkable changes when treated at 200 degrees C in sodium chloride solution and artificial sea-water environments. Even though treated in Na-bearing reaction solution, they did not always produce analcime. It is possible to estimate from the present experiments that the alkalinity of the reaction solution is a significant controlling factor in analcimization, and the possible formation temperature of the analcime zone cannot exceed 200 degrees C. It also appears probable that the analcime zone around Kuroko-type mineral deposits is formed by the interaction of an alkaline interstitial solution and the precursor zeolite-bearing acidic pyroclastic rocks. These interstitial solutions might have been heated by submarine volcanism and/or exhalative activities which accelerated analcimization.