After last week's stock market roller coaster, says New York investment banker Felix Rohatyn, things will never be the same.

"I'm not sure what we learned because it's all been so dizzying," said the Lazard Freres senior partner, who has 40 years experience in business and on Wall Street.

"But one thing I've concluded is that the stock market as we knew it no longer exists. When the market can go down 700 points {on the Dow Jones industrial index} in three days, and then come back almost 300 points in two days, as it did on Tuesday and Wednesday, it's no longer a marketplace as we knew it where people could put investments and feel reasonably secure."

Rohatyn is right: In this situation, the market more closely resembles a crap game, in which all bets can be won or lost on a single roll of the dice.

Behind Rohatyn's concern is a simple, harsh fact of life with which investors will have to contend: Since the stock market dropped 508 points in one day -- beyond anyone's worst dream -- it can happen again. There is also the potential for upswings of a couple hundred points in a day or a few hours.

"This is a totally new environment," observes former State Department official Robert Hormats, now a Goldman, Sachs & Co. executive. "We've generally assumed there would be stability in the stock market. We can't assume that anymore."

Just think of how easily we have become accustomed to big swings in the market: After Monday's drop of 508 points, Tuesday's 102-point recovery caused barely a lifted eyebrow. Yet, that was the biggest single-day's advance ever -- and it covered only 20 percent of the Monday crash. The record lasted exactly one day: on Wednesday, the index was up 187 points!

As recently as Jan. 8, 1986, when the Dow average dropped 39 points, it was a shock that caused the first serious questioning of the impact of computerized program trading, the new Wall Street game that guarantees big bettors millions in profits with no risk.

Referring then to that 39-point loss, John Montgomery, president of the Washington Society of Investment Analysts, said, "You get a couple of back-to-back days like that and people will start to scream."

How times have changed! During the wild gyrations on Monday and Tuesday, the Dow index frequently shifted more than 39 points in 39 minutes. There were 10- and 20-point changes in individual stock prices within hours -- moves that in the past might have taken days or months.

In some cases, the gap was so large between the price that sellers asked and the price that buyers were willing to pay that individual stocks were not traded for hours, or were not allowed by the market authorities to be bought and sold at all.

"That's no longer a marketplace as we knew it. I don't know what to call it. Is it a casino?" Rohatyn asked. "I find this frightening, beyond anything we've experienced. We should deal with it, stabilize it and make sure that it never happens again."

Salomon Brothers economist Henry Kaufman and Harvard professor Robert Reich agree with Rohatyn that one of the basic reasons for the new volatility in markets -- it has been evident in exchange rates and bonds as well, although both those markets behaved relatively well last week -- is the drift to financial deregulation. The introduction of new-fangled "financial instruments" has encouraged wheeling and dealing at the expense of stability in markets.

For example, the growing use of computerized trading, sometimes called stock index arbitrage, clearly has tended to push prices up faster and further in bull markets, and accentuated the decline when things are going the other way. When interacting with a newer, sophisticated management tool called portfolio insurance, these forces helped generate a free fall in prices on Monday.

"We have to limit the use of destabilizing financial 'Star Wars' that have spawned computer program trading and portfolio insurance," Rohatyn said.

After the Monday debacle, the New York Stock Exchange for the first time limited program trading, and the Chicago Mercantile Exchange put daily limits on stock index futures prices, in a way comparable to limits that exist now for cotton and other commodity futures.

Kaufman and Rohatyn have argued for years that under Reagan, the financial markets have been given too much unbridled power in the name of deregulation. Speculators have been able to use junk bonds, with little or no cash backing, to take over going corporations. The takeover mania has been one of the driving forces behind the mad upward rush of stock prices: As the markets learned that this or that company was a takeover candidate, share prices boomed.

As the week came to an end, some traders weren't sure that the quick and sizable upturn from the low point was all that healthy. There was a palpable yearning for a semblance of stability, rather than a complete recovery.

As Wall Street consultant Sam Nakagama put it: "I'd hate to see the markets come back too fast -- I'd rather see them bumble around." Nakagama's point is that it wouldn't be a bad result if the stock market decline causes a contraction in consumer spending. That would slow down the economy modestly, and reduce the trade deficit. Besides, too quick a recovery will erase the incentive for President Reagan to work out a compromise with Congress on a deficit-reduction package -- a "must" agreed on by almost everyone.