Justice Department prosecutors and Securities and Exchange Commission regulators are investigating Merck & Co. for its handling of the painkiller Vioxx, the pharmaceutical giant disclosed yesterday.

Merck, based in Whitehouse Station, N.J., said in an SEC quarterly filing that both inquiries are in their early stages and focus on its blockbuster arthritis painkiller, which was pulled off the market Sept. 30 after a study found long-term use of it doubled the risk of heart attacks and strokes.

Merck described the SEC's probe as an "informal inquiry," which generally means that the SEC has not yet sent out subpoenas. SEC investigations tend to focus on whether public companies were forthright with investors about potential problems or whether insiders somehow profited from advance notice of company news.

The Justice Department inquiry has more serious potential implications for Merck because it is a criminal probe. The company said it has received a subpoena "requesting information related to the Company's research, marketing and selling activities with respect to Vioxx in a federal healthcare investigation under criminal statutes."

In recent years, federal prosecutors have gone after other pharmaceutical companies for improperly promoting drugs for unapproved -- known as "off-label" -- uses and for offering doctors improper inducements to prescribe specific drugs. Federal law also makes it illegal to conceal side effects or otherwise mislead the public about a drug's safety. But such cases are difficult to prove and have rarely been brought, legal experts said.

"Merck acted responsibly and appropriately as it developed and marketed Vioxx," a company spokeswoman said yesterday. "Merck will cooperate with the Justice Department."

The SEC and the Justice Department have policies against discussing ongoing investigations.

Merck also said that as of Oct. 31, it had been named in about 375 lawsuits representing 1,000 plaintiff groups alleging they suffered side effects from Vioxx. State judges in California and New Jersey have already ruled that cases there can proceed as a group or class action. Merck said in the filing that it is seeking to consolidate all of the federal cases before a single judge.

Merck's stock price has fallen more than 40 percent since Vioxx was withdrawn.