CVS Health Corp. kicked off the third-largest corporate bond sale on record to fund its acquisition of Aetna, testing the appetite of a market that's off to its worst annual start in decades.

The pharmacy giant plans to issue $40 billion of the investment-grade debt in a nine-part offering, people with knowledge of the matter said. That would be surpassed only by Verizon Communication's $49 billion offering in 2013 and Anheuser-Busch InBev's $46 billion sale two years ago.

The blockbuster sale is a big test for whether the corporate-bond market can maintain the insatiable demand seen in recent years as yield seekers found few options elsewhere. With the Federal Reserve hiking interest rates and withdrawing its unprecedented stimulus measures - albeit at a slow pace - yields on investment-grade debt have climbed to the highest levels in six years.

Such a massive offering at juicier yields is attracting interest from investors who had previously been staying on the sidelines. CVS said in a Feb. 26 presentation it was targeting as much as $44.8 billion of new debt, but ended up offering $40 billion -- even as bankers received orders for three times that amount. It was important to sell the deal at attractive prices to entice investors in a market that's been on shaky footing this year, according to Matt Brill, a senior portfolio manager at Invesco Ltd.

"We've been saving all of our chips for this," said Brill, who helped oversee around $226 billion of fixed-income assets as of the end of 2017. "This deal awoke the sleeping giant. It needs to go well to get the ball rolling again and get people more interested in corporate credit again."

CVS is selling fixed and floating-rate senior unsecured bonds in as many as nine parts. The longest portion of the offering, a 30-year security, may yield 1.95 percentage points above Treasuries, after initial talk of around 2.15 percentage points, the person said, asking not to be identified as the details are private. CVS gathered $49 billion in bridge loans from 20 lenders in December as temporary financing for the acquisition, which it is looking to refinance with Tuesday's transaction.

"This looks like a reasonably attractive entry point for investors," said Travis King, head of investment-grade credit at Voya Investment Management in Atlanta. Given that CVS will make up a much greater percentage of investment-grade indexes after the transaction, "it's a very difficult deal to be underweight on right out of the gate. That helps support it overall."

Yields on high-grade bonds have risen half a percentage point since the start of 2018, pushing up the cost of debt financing and lowering issuance overall. With those price declines, investment grade bonds returned minus 2.6 percent in the first two months of the year, making for the worst start for the asset class since 1981. With average yields still relatively close to historic lows, companies are continuing to turn to debt markets to fund their acquisitions.

CVS is trying to win regulatory approval for its plan to buy Aetna for $67.5 billion, which will bring together around 10,000 CVS stores and Aetna's 22 million customers. The deal is among the biggest health-care mergers of the past decade and would create a behemoth that will try to shift some of Aetna customers' care away from doctors and hospitals and into thousands of CVS stores. The merger is expected to close in the second half of this year.

"Managed-care companies on their own offer limited value in today's world of high-deductible health plans," said Bloomberg Intelligence analyst Mike Holland. "It's a good save for Aetna and for CVS, it gives them a much more stable revenue stream."

Working in CVS's favor is that most investors will view it as a health-care company instead of a retail one, said Mickey Chadha, an analyst at Moody's Investors Service. The completion of the acquisition will likely result in a one-notch downgrade to the current rating that at Baa1 is three steps above speculative-grade, Chadha said.

"It's a strategic, sound transaction for the longer-term, but CVS has to integrate another company that has not necessarily been done before between a retailer and an insurance company," he said. "There are elevated risks in terms of that execution."

S&P Global Ratings downgraded CVS one level to BBB, its second-lowest investment-grade rating following the announcement of the debt offering.

Verizon set the record for corporate bond sales in 2013 with its $49 billion offering to buy out Vodafone Group's stake in Verizon Wireless. The second biggest was Anheuser-Busch's $46 billion sale to purchase SABMiller in 2016. Broadcom last month lined up as much as $106 billion of debt financing to back its proposed acquisition of Qualcomm, but the target company has rejected several offers and no debt has been offered yet.

Barclays Plc, Goldman Sachs Group Inc., Bank of America Corp., JPMorgan Chase & Co. and Wells Fargo & Co. are managing the bond sale, CVS said in a filing.