1988. Reflecting these price movements, the dollar value of petroleum imports rose from about US$2.8 billion in 1970 to nearly US$58 billion in 1980, and then fell a low of US$26 billion in 1988 before making a slight recovery to US$41 billion in 1990. A third factor affecting imports was trade liberalization. Reduced tariff rates and a weakening of other overt trade barriers meant that imports should have been able to compete more fully in Japan's markets. The extent to which this was true, however, was subject to much debate among analysts. The share of manufactured imports in GNP changed very little from 1970 to 1985, suggesting that falling import barriers had little impact on the propensity to purchase foreign products. Falling trade barriers might become more significant in the 1990s as liberalization continues. Yet another factor determining import levels was the exchange rate. After the ending of the Bretton Woods System in 1971, the yen appreciated against the United States dollar and other currencies. The appreciation of the yen made imports less expensive to Japan, but it had a complex effect on total imports. Demand for raw material imports was not affected much by price changes (at least in the short run). Demand for manufactured goods, however, was more responsive to price changes. Much of the rapid increase in imports of manufactures after 1985, when the yen began to appreciate rapidly, can be attributed to this exchange-rate effect. All factors combined led to the rapid growth of imports in the 1960s and 1970s and their very slow growth in the 1980s. Rapid economic growth combined with stable import prices and the shift toward imported raw materials brought high import growth in the 1960s. The big jump in raw material prices in the 1970s kept import growth high despite lower economic growth. In the 1980s, falling raw material prices, a relatively weak yen, and continued modest economic growth kept import growth low in the first half of the decade. Import growth finally accelerated in the second half of the 1980s, when raw material prices stopped falling and as the rise in the value of the yen encouraged manufactured imports. Japan imported a wide range of products, although energy sources, raw materials, and food were the major items. Mineral fuels, for example, rose from under 17% of all imports in 1960 to a high of nearly 50% in 1980. They had