The value of these currencies rarely rises or falls on the basis of how much foreign exchange reserves the Central Bank has accumulated, but rather on corporate indicators, economic fundamentals and capital market movements. Some central banks, through open market operations aimed at preventing their currency from appreciating, can at the same time build substantial reserves.
For many years Japan engaged in a  2  BIS Review 1/2006  programme of buying US dollars in order to keep the yen/US dollar rate down so as to make its exports more competitive. This also led to a massive build up in foreign exchange reserves in the Bank of Japan.