Pharmaceutical giant Pfizer Inc. (PFE) launched its $13.5 billion, multi-part offering in the U.S. corporate bond market to help fund its $68 billion acquisition of health-care company Wyeth (WYE).

Pfizer's offering, rated triple-A by Standard & Poor's and two notches lower by Moody's Investors Service, is scheduled to be sold later Tuesday, according to a person familiar with the deal.

Investors, who have been clamoring for investment-grade bonds this year from companies with little debt and those that can best weather economic downturns, swooped in for the Pfizer offering. This helped narrow risk premiums on the bulk of the bonds from original expectations.

Pfizer's deal is the latest in the sector to tap the U.S. high-grade market for funds amid a flurry of merger and acquisition activity in the pharmaceutical industry. Last month, Roche Holding AG (RHHBY) raised a record $16.5 billion in the bond market to finance its purchase of Genentech Inc. (DNA). Pfizer's offering is the second-largest U.S.-dollar denominated deal, according to data firm Dealogic.

"Investors are looking for investment-grade names even though we have had good supply," said Daniel Sheppard, director at Deutsche Bank Private Wealth Management. "There's still good appetite for such paper."

With banks' lending capabilities limited, many companies are choosing to tap hungry bond investors for funds to refinance portions of temporary financing, known as bridge facilities, sooner rather than later. Pfizer completed syndication of its bridge loan only at the end of last week. In other cases, companies are skipping the bridge loan altogether. Roche, for example, opted to pre-fund its acquisition in the bond market first before securing financing in the loan market.

And there's more big pharma debt on tap. Merck & Co. (MRK) plans to sell bonds to partly refinance its $8.5 billion bridge loan secured to buy rival Schering-Plough Corp. (SGP). Merck agreed last week to buy Schering-Plough for $41.1 billion.

The consolidation in the pharmaceutical industry is being driven by a wave of expirations of patents for blockbuster drugs, which are exposing them to competition from cheaper generic versions. At the same time, companies have hit numerous setbacks in recent years finding successful new drugs to replace the lost revenue.

The rationale behind the combinations is to generate big cost savings, beef up pipelines of experimental drugs, and to diversify into areas outside of traditional prescription drugs, such as consumer health products and biotechnology-style drugs. What remains an open question is whether the latest round of consolidation will avoid the same fate of some industry mega-mergers earlier in this decade, which disrupted research efforts and hurt stock price performance.

Pfizer's shares were recently down 17 cents, or 1.2%, at $13.98.

Pfizer's two-year, floating-rate $1.25 billion notes launched at 195 basis points over the three-month London interbank offered rate, or Libor, according to a source familiar with the deal.

The $3.5 billion, three-year, fixed-rate bonds launched at 305 basis points over Treasurys. Guidance was in the 310 basis points over Treasurys area.

The $3 billion, six-year portion launched at 340 basis points over Treasurys. Guidance was in the 345 basis points over Treasurys area.

The $3.25 billion, 10-year part launched at 325 basis points over Treasurys versus guidance of 330 basis points over Treasurys area.

The $2.5 billion, 30-year tranche launched 345 basis points over Treasurys versus guidance of 350 basis-point area.

Joint leads on the deal are Citigroup (C), Barclays (BCS), Bank of America/Merrill Lynch (BAC) Goldman Sachs (GS) and JPMorgan (JPM).

Pfizer spokeswoman Joan Campion on Tuesday declined to comment beyond a Monday statement, which said: "We had previously said we may issue bonds to replace much of the bridge financing to secure more longer-term debt."

Pfizer last sold a $1.45 billion deal on Jan. 27, 2004, according to Dealogic.

-By Anusha Shrivastava and Romy Varghese, Dow Jones Newswires; 201-938-2371; anusha.shrivastava@dowjones.com

(Kate Haywood and Peter Loftus contributed to this report.)