TOKYO (AP) _ Share prices posted moderate gains in morning trading on the Tokyo Stock Exchange Wednesday and the dollar changed little from its higher opening against the yen.

The 225-issue Nikkei Stock Average gained 55.45 points, or 0.23 percent, to close the morning session at 23,499.06. The index was off about 5 points in early afternoon trading. On Tuesday, the average rose 309.18 points, or 1.34 percent.

The Tokyo Stock Price Index of all issues listed in the market's first section stood at 1,809.05 points, up 1.34 points.

At late morning, the dollar was changing at 133.92 yen, up 0.29 yen from Tuesday's close. It opened at 133.93 yen after finishing overnight at 133.85 yen in New York.

Benchmark No. 129 10-year Japanese government bonds edged lower at 101.59 points by late morning from Tuesday's 101.64-point finish. Their yield rose to 6.115 percent from 6.105 percent.

Shigeki Sato of the stock information department with Okasan Securities said the market's main barometer was supported by active buying by foreign investors and arbitrage trading.

But he said no specific news was moving the market.

Other traders said that despite an overnight decline on Wall Street and the yen's slip against the dollar, stock trading remained substantial with volume on the first section totaling an estimated 300 million shares in the morning session.

Meanwhile, the dollar started to slip after an initial boost of buying to cover short positions but later inched back toward its opening level, traders said.

''Dollar trading is static. There are no customers,'' said Tadasu Ochi, a dealer with Mitsubishi Bank.

The dollar has been falling steadily since the U.S. Federal Reserve Board lowered its key lending rate Friday in an attempt to stimulate the economy.

Lower interest rates tend to weaken a nation's currency by making it less attractive to investors.

Most market players do not expect the Fed to further lower interest rates immediately but are still reluctant to buy the dollar, Ochi said.