The case is one of the most contentious to emerge from the collapse of Enron. In it, J. P. Morgan is asking a court to force 11 insurance companies to pay about $1 billion to meet the terms of surety bonds that were issued to back oil and gas trades involving Enron.

The insurers have countered that the transactions were not true commodity deals but rather secret loans from the bank to Enron constructed by using an offshore entity tied to J. P. Morgan known as Mahonia Ltd.

This was accomplished, they contend, by a circle of transactions that canceled out every element of the deals except for a debt owed by Enron. That money would have gone to J. P. Morgan had Enron not filed for bankruptcy.

J. P. Morgan tried to have the e-mail messages — which were written by its vice chairman, Donald Layton — excluded, essentially contending that the colloquial meaning of the words do not convey what Mr. Layton intended.

But in his 10-page ruling, the federal district judge, Jed Rakoff, dismissed the bank's position, saying that the meaning of the words in the e-mail messages was a question of fact for the jury to determine.

"If the jury accepts defendants' view of what the e-mails are referring to (as the jury reasonably might), the term `disguised loan' is highly relevant and precisely descriptive of what is involved," Judge Rakoff wrote.

The e-mail messages were written over different months in 1999 and were part of a long-running discussion within J. P. Morgan about the appropriate way to treat transactions of the type that were entered into by Enron. The e-mail messages indicate that J. P. Morgan engaged in similar deals with a raft of companies around the globe.

"We are making disguised loans, usually buried in commodities or equities derivatives (and I'm sure in other areas)," Mr. Layton wrote in one of the e-mail messages. "With a few exceptions, they are understood to be disguised loans and approved as such. But I am queasy about the process."

In testimony given last week outside the jury's presence and cited in Judge Rakoff's decision, Mr. Layton said that he used the term "disguised loans" to mean that the transactions involved cash advances from the bank, but without the controls placed on a normal loan. In essence, he testified, the term was meant to indicate that the loans may be disguised from the bank itself, not from others.

Indeed, Mr. Layton worried in the e-mail message that J. P. Morgan itself might not be able to keep track of its loan exposure because the transactions were "buried in the trading books." The inability of investors to decipher Enron's true financial condition because of deals like the ones structured by J. P. Morgan has been a central issue in the examinations of the company's collapse.

A principal question in the case has been whether the insurers knew the true nature of the transactions when they issued the surety bonds, as J. P. Morgan contends, which would arguably make them liable for payment. But the insurers counter that they never would have provided bonds for the transactions if they had known them to be loans.