One commissioner, Harvey J. Goldschmid, questioned the staff about the consequences of the proposed exemption. It would only be available for the largest firms, he was reassuringly told — those with assets greater than $5 billion.

“We’ve said these are the big guys,” Mr. Goldschmid said, provoking nervous laughter, “but that means if anything goes wrong, it’s going to be an awfully big mess.”

Mr. Goldschmid, an authority on securities law from Columbia, was a behind-the-scenes adviser in 2002 to Senator Paul S. Sarbanes when he rewrote the nation’s corporate laws after a wave of accounting scandals. “Do we feel secure if there are these drops in capital we really will have investor protection?” Mr. Goldschmid asked. A senior staff member said the commission would hire the best minds, including people with strong quantitative skills to parse the banks’ balance sheets.

Annette L. Nazareth, the head of market regulation, reassured the commission that under the new rules, the companies for the first time could be restricted by the commission from excessively risky activity. She was later appointed a commissioner and served until January 2008.

“I’m very happy to support it,” said Commissioner Roel C. Campos, a former federal prosecutor and owner of a small radio broadcasting company from Houston, who then deadpanned: “And I keep my fingers crossed for the future.”

The proceeding was sparsely attended. None of the major media outlets, including The New York Times, covered it.

After 55 minutes of discussion, which can now be heard on the Web sites of the agency and The Times, the chairman, William H. Donaldson, a veteran Wall Street executive, called for a vote. It was unanimous. The decision, changing what was known as the net capital rule, was completed and published in The Federal Register a few months later.