Elizabeth Rose, a specialist with Lehman Brothers MarketMakers, works her post Monday on the trading floor of the New York Stock Exchange. A stunning reshaping of the Wall Street landscape sent stocks down sharply Monday.

San Francisco ? Once again, stockholders are getting a harsh reminder that the worst time to make an investment decision is when markets are in crisis.

The dramatic collapse of Lehman Brothers and the hurried sale of Merrill Lynch over the weekend saddled investors with even steeper losses on Monday, but securities regulators moved quickly to reassure individuals that their brokerage accounts are safe, and investing veterans said that perspective, not panic, is in order.

“It’s too late to panic,” said Zachary Karabell, president of independent consulting firm River Twice Research. “And if it’s too late to panic, it’s probably time to calmly look at what’s going on in light of opportunities.”

“Selling doesn’t recoup your losses,” he added. “It just realizes them.”

When an institution of Lehman’s size and clout goes under, it’s understandable to wonder if your money is safe.

In a word, yes.

“I don’t think anybody’s money in accounts is at risk,” Karabell said. “There’s no reason to be running down to your bank or phoning your broker.”

Customers of credit unions also are protected in a worst-case scenario, said Daniel Penrod, an analyst for the California and Nevada Credit Union League, an industry trade association.

As with the FDIC insurance that guarantees bank deposits, credit union savings are insured up to $100,000 per account and $250,000 for individual retirement accounts.

“While credit unions were not part of the disease, we’re definitely dealing with the symptoms,” Penrod said. “But for the most part credit unions are showing great strength in their capital reserves.”

Regulators also looked to keep investors calm. The Securities and Exchange Commission said it’s taking steps to ensure that Lehman clients will not be adversely affected by the firm’s collapse.

The SEC said the Securities Investor Protection Corporation, a four-decades-old insurance plan for brokerage accounts, would cover the broker-dealer’s customers in a worst-case scenario. SIPC rules provide coverage up to a maximum of $500,000 per client, including a maximum of $100,000 for cash.

SIPC has not had to safeguard Lehman accounts, said Stephen Harbeck, SIPC president and chief executive.

“This demonstrates that the system works,” he said. “Customer assets have been properly segregated at Lehman Brothers; that’s why there’s been no need for SIPC to take action, but if the facts change, we’re ready.”

In announcing bankruptcy, Lehman made it clear that only the holding company was making the filing. Shareholders in Neuberger Berman, the firm’s mutual-fund unit, and Lehman Brothers Asset Management will not be affected, Lehman said.

Separately, Merrill Lynch has agreed to be acquired by Bank of America for $50 billion.

Merrill Lynch’s customer accounts shouldn’t be affected by that transaction, as the deal is a straightforward acquisition.