Bonds have been on fire during the past year, as investors regained their confidence after the financial crisis ended.
The Barclays Capital long-term corporate bond index produced a return of 31 percent during the last 52 weeks.
But many experts say the good times are about to end.
The burgeoning budget deficit and government debt burden are a big factor, they say.
The deficit is projected at $1.5 trillion for this year.
The Federal Reserve’s accommodative monetary policy also will hurt bond prices, many maintain.
"This is the last leg of money coming off the sidelines, the final rotation into the bond market," Bob Froelich, senior managing director at The Hartford, told CNBC.
"This is the beginning of the end. The bond market is a bubble. It's getting ready to burst."
In addition to keeping interest rates at record lows, the Fed has intervened directly in the bond market, buying Treasuries and mortgage securities.
"You have the Fed to thank for this," Greg Habeeb, head of the taxable bonds at Calvert Investments, told CNBC.
"With their determination to save the financial system, they've had their party, and they've created another bubble, which we call the credit bubble."
Others are bearish on bonds too.
“We’re going to have supply (to fund the budget deficit), and recent auctions were not so good,” Kei Katayama, head of fixed income at Daiwa SB Investments, told Bloomberg.
“There’s some concern yields will go higher.”
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