In 1975 amendments to the Securities Exchange Act of 1934, Congress called for the establishment of a national market system. By focusing on the needs of investors rather than dictating a specific market structure, the 1975 amendments established permanent goals for which securities markets should strive. The Securities and Exchange Commission (SEC) is responsible for monitoring market structure to assure the amendments' goals are continually addressed. The GAO analysis concludes that the SEC needs to re-evaluate several issues regarding the securities market structure to insure the market meets national goals. Additionally, the SEC needs to reopen the trading restriction issue, since trading restrictions help maintain the market structure. Removing such restrictions may substantially alter the way exchange-listed securities are traded, including the possible elimination of trading floors and specialists. Despite substantial changes in the market since 1980, no comprehensive SEC reviews have been conducted to demonstrate whether remaining trading restrictions impair market efficiency, fairness, or competitiveness. A comprehensive review of the intermarket trading system is needed, as well as the development of trading linkages for multiple listing of options under SEC monitoring and guidance. Appendixes contain further information on U.S. securities markets. 4 tables, 2 figures.