An epic trade that would have made Chris Paul and Kobe Bryant teammates in Los Angeles was vetoed Thursday night by N.B.A. Commissioner David Stern in a decision that rocked the league just as it emerged from a five-month lockout.

The New Orleans Hornets had agreed to send Paul, a four-time All-Star, to the Lakers in a three-team deal that involved the Houston Rockets. Only 45 minutes after the teams reached an agreement, Stern called the Hornets and vetoed the deal, according to a person involved in the deal.

“The league has killed it,” said the person, who spoke on condition of anonymity to avoid jeopardizing his role in the talks. “I know that’s 100 percent true.”

Asked who had made the decision, the person said, “It definitely was David.”

The league did not immediately explain the decision, other than to issue a terse statement through a spokesman, saying, “League office declined to make the trade for basketball reasons.”