Philippine central bank Governor Rafael Buenaventura told Reuters that, contrary to some dealers' suspicions, the bank didn't intervene to buy pesos on Wednesday.
Moody's downgrade, which took the Philippines' rating four levels below investment grade, came two days after the Philippine central bank governor said he expected Moody's to lower it by only one notch.
The peso, Asia's best performing currency this year, was strong in morning trades before the rating downgrade, as it recovered from a decline on Tuesday in the wake of bomb blasts in Manila's business district and other parts of the country.
The economy expanded at its fastest pace in 15 years in 2004.
The South Korean won gained as much as a third of a percent to 1,023.5 per dollar in local trades, its strongest since November 1997. Foreign investors were net buyers of South Korean shares for a fourth straight session on Tuesday.
The Taiwan dollar gained as much as a fifth of a percent to 31.52 per dollar, nearing a four-year high of 31.50 reached in December. Data showed foreign investors were net buyers of Taiwanese shares for a ninth straight session on Tuesday.
The sustained equity inflows into Asia are likely to buoy the region's currencies against the dollar, said Irene Cheung, head of Asian currency strategy at ABN Amro Bank in Singapore.
Still, the currency gains could be checked by central bank dollar purchases, she said in a note to clients.
Other Asian currencies were little changed ahead of US Federal Reserve Chairman Alan Greenspan's testimony to the Congress on Thursday. Traders are looking for signs of whether the Fed will hasten the pace of increases in US interest rates.
The dollar has risen 2.1 percent against the yen and 4.2 percent against the euro this year on expectations of higher US interest rates.