BUDAPEST, JUNE 21 -- The first stock exchange in post-Communist Eastern Europe opened formally today in Budapest, 42 years after it was closed by Communist authorities.

Officials hope the fledgling Budapest trading floor will boost market-oriented economic reforms and become the stock exchange center for all of Eastern Europe.

On its first day, the exchange reported 58.1 million forints, more than $900,000, worth of trading in the 41 listed companies.

Prime Minister Jozsef Antall, in a telegram from West Germany where he is on an official visit, said the Budapest Stock Exchange is ''an important institution of a market economy.''

Richard C. Breeden, chairman of the U.S. Securities and Exchange Commission, presented the exchange with a new bell modeled after one used for years to open trading on the New York Stock Exchange.

A stock exchange first opened in Hungary in 1864 but was closed in 1948 when Communists came to power after World War II.

''Until the '80s it was heresy to even suggest anything of the sort,'' said Hungary's interim president, Arpad Goncz, alluding to Communists' description of a stock market as the ultimate symbol of capitalism.

In March, a key government act on the stock exchange and securities took effect, giving the market a legal framework.

But financial experts said the stock exchange still has a long way to go before it establishes itself on the international market.

Finance Minister Ferenc Rabar said at the opening ceremony that the exchange ''does not in itself solve the problem of the liberalization of the economy.'' He said ''we have much to do yet in that regard.''

The exchange's main aim is to achieve reprivatization of the country's inefficient, state-owned enterprises by allowing companies to sell shares to meet their capital needs.