In a display of what one observer dubbed "fin-de-siecle madness," some financial services companies with Internet intentions enjoyed dizzying stock price run-ups followed by swift selloffs this week.

Analysts attributed the fluctuations to Internet-crazed day traders- individual investors who concentrate on short-term trades.

Sovereign Bancorp of Wyomissing, Pa., said Wednesday morning that it had met with analysts to discuss forming a separate Internet bank. The news sent the company's shares soaring as high as $26.25 during the day, and plunging as low as $14.375.

The stock ended the session at $14.6875, well below Tuesday's close of $17.50. Over 40 million shares traded hands, versus a daily average of less than a million.

Also Wednesday, North Fork Bancorp. of Melville, N.Y., said it plans to expand its Internet bank activities. In reaction, the company's shares rocketed to $26.75 after a long opening delay, then traded down as low as $24.25 in less than an hour. North Fork closed at $23.625

In another episode, Florida Banks Inc. of Jacksonville, after posting a first-quarter net loss of $224,754 on Monday, said Tuesday that it plans to offer real-time Internet banking. Its stock price more than tripled that day, closing at $31.25. But Wednesday, shares of the $145 million-asset bank closed down more than $14.25, at $17.

Stocks of established Internet-only banks and nonbanking companies also endured wild rides this week.

Shares of IndyMac Mortgage Holdings Inc., a real estate investment trust, bounded as high as $16.75 on Wednesday-up more than 30% from Tuesday's close. The jump occurred after the company said it would originate home improvement loans through the Internet. The stock closed Wednesday at $13.875, up $2.0625 for the day.

And in another case, shares of First Sierra Financial Inc., a computer leasing company that said Wednesday morning that it was setting up an Internet bank, zoomed more than 50% in price. The company closed at $19.75, up $1 for the day.

Veteran bank analysts said the price swings made little sense.

"Most banks have already been doing Internet banking," said Nancy Bush, an analyst with Ryan, Beck & Co.

"Some are very active on the Web," Ms. Bush said, "but no one is making money at it."

Analysts said the day traders responsible for the stock spikes could make the general public wary of Wall Street investments. The day traders themselves would thus wind up losing money, analysts said.

"A bunch of guys who two months ago were working in a big-and-tall mens' shop are sitting at home in their bathrobes watching C-Span and pretending that they are George Soros," said Sean J. Ryan, a Bear, Stearns & Co. analyst.

Mr. Ryan predicted "financial ruin" for a number of unsophisticated investors who "think they are market gurus." He added, though, that he may be biased. "I missed out on the Internet bubble," he said. "Maybe that why I'm so caustic."

Some analysts criticized the companies themselves, saying they were trying to take advantage of Internet madness.

"Put out a press release, change your ticker symbol, and you're off to the races," said Fox-Pitt, Kelton analyst Reilly Tierney.

While he raised questions about the profitability of First Sierra, some investors defended the company. "They're using the Internet to create a business to business solution," said one portfolio manager.

Sovereign Bank chief executive Jay S. Sidhu dubbed the market activity "crazy" and said his company had not made its announcement "just for the sake of a hike in the stock price."

The Internet is a "very viable delivery system" that reaches the "high- income customers" Sovereign caters to, Mr. Sidhu said.

Meanwhile, shares of Finova Group Inc. fell $2. 375 on Wednesday, to $47.50, after the Phoenix commercial finance company delayed its earnings release.

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