It's time to amend the old rule that "What goes up, must come down." Based on the bloody events in the stock market during the past two weeks, the new rule should be, "What goes up fastest, comes down fastest."

The proof is to be found in a comparison between the gains and losses of the 30 stocks in the Dow Jones industrial average, a national blue-chip list, and the 30 stocks in the Johnston, Lemon & Co. index, a Washington area blue-chip list.

The comparison shows the following:

While the Dow stocks soared higher and higher for most of the year, the J&L stocks lagged behind. As of Aug. 21, just before the Dow reached its peak, the Dow industrials were up 43 percent for the year while the J&L stocks were up only 26 percent -- a difference of 17 percentage points.

But then came the huge waves of selling that wreaked havoc on stock prices. In the process, the Dow stocks lost almost half of the gains they made since January.

By Friday, after one of the worst slides in history, the Dow stocks were still up 18.5 percent for the year, while the J&L stocks were still up 14 percent.

Thus, the gap between the two groups had narrowed from 17 percentage points to only 4.5.

Looking at it another way, the Dow hit its all-time high of 2722.42 on Aug. 25. Since then, the trend has been downhill, capped by the Dow's staggering loss of 108 points on Friday to close the week at 2246.73. From peak to valley, the value of the Dow stocks declined 17.5 percent in the last eight weeks.

In the same period, the J&L stocks dropped only 10.5 percent.

The Dow stocks, the fastest to rise, were the fastest to fall. The J&L stocks, having gone up less, dropped less.

It is worth noting that that pattern was solidly intact up until Friday. But as the selloff deepened, more and more secondary stocks were getting hit.

In short, it began to look as though investors were selling almost any stock in which they had a profit, squeezing even minor gains out of the shares.

The J&L index includes the following stocks:

Avemco Corp., Ameribanc Investors, BDM International, B.F. Saul Real Estate Investment Trust, Comsat, Dart Group, Fairchild Industries, Federal National Mortgage, First Virginia Banks and Gannett Corp.

Also, Geico Corp., Giant Food, Hechinger Co. B, Kay Jewelers, Manor Care, Marriott Corp., MNC Financial, Martin Marietta, MCI Communications Corp., Norfolk Southern, Perpetual Savings Bank and Pepco.

Also, Riggs National Bank, Student Loan Marketing Association, UNC Inc., USLICO Corp., USAir Group, Washington Gas Light, Washington Post and Washington Real Estate Investment Trust.

While the price of those 30 J&L stocks declined an average of 10.5 percent between Aug. 25 and last Friday, some stocks did significantly better or worse.

The largest losers in the eight-week period included Federal National Mortgage, known as Fannie Mae, which is sensitive to changes in interest rates. It dropped 24 percent.

Giant Food also dropped 24 percent, losing $4.75 a share on Friday. Hechinger B stock was down almost 24 percent, as was USAir Group, which dropped $2.50 on Friday.

Other stocks that took it on the chin in the recent slide were Norfolk Southern, down 22.7 percent; Marriott Corp., down 15 percent; BDM International, down 14 percent; Washington Gas Light, down 13.3 percent; MNC Financial Corp., down 12.4 percent; Martin Marietta, down 10.3 percent, and Washington Post, down almost 10 percent.

Stocks that didn't lose heavily were chiefly those shares that hadn't gained much recently. That was especially true of the banks -- First Virginia, Perpetual Savings and Riggs National.

One stock that was unscathed in the market's turmoil was MCI Communications Corp., which has been a strong performer of late. It was up 6.4 percent since August.

Stocks that survived with only minor losses were insurance companies -- Avenco, down 3.3 percent, and Geico, which lost only about 5 percent.

For the investor interested in value-oriented stocks that do well over the long term, the huge gap between the performance of the Dow stocks, such as IBM, GE and Merck, and the smaller Washington area stocks has been mystifying and annoying. But it was clear that institutional and foreign investors thought the Dow stocks were the safest place to be.

Many analysts predicted that when the market turned down, the biggest gainers would be the biggest losers. And that has turned out to be true.

The slide has closed the gap between the Dow stocks and the Washington area stocks. But most investors would have preferred to see local stocks move up, instead of Dow stocks moving down.

What was the mood in the office of a typical retail stock broker in Washington last week as the market headed south? As one broker for a national firm told it, customers were calling, often with fear in their voices, to ask: "What shall I do?" And, for the most part, the customers were being told not to sell -- that the market's downturn was a correction, not a crash.

That advice was coming chiefly from the firm's New York analysts, who talk to brokers on a nationwide "squawk box" and were saying, essentially, "Don't panic. Don't sell. The fundamentals of the market haven't changed. The correction is a normal event."

"We hope they are right," said the broker, noting that despite all the warnings from New York about getting caught up by emotionalism, he was finding it very tough to hang tough.

"A lot of people believe this is the end of the bull market and they don't want to own brokerage stocks."

That attitude, said Chip Mason, head of Legg Mason, the Baltimore brokerage house, is one reason brokerage house shares have been declining in recent months. Another reason is that many brokerages suffered substantial losses in the bond market this year. But, said Mason, the decline in his firm's share price has been in line with the drop in the stocks of rival firms.

"It's right down the line," he said.

Legg Mason stock has fallen from a high of about $24 earlier this year to $13.75 Friday. Alex. Brown & Sons of Baltimore, once at the $28 level, ended the week at $15.25. And Scott & Stringfellow of Richmond, which has traded as high as $15.50 in the last year, closed at a new low of $10.75.

A key Washington high-tech stock got a bit of a haircut in the market's downturn last week. Atlantic Research Corp. fell to $26 Friday from $28.25 a week ago.

Analyst Mae G. O'Leary of Baker, Watts & Co., noted that until the stock got hit, ARC shares had been on the rise. A key reason was that Clabir Corp. of Connecticut recently said it would give up its takeover bid for ARC. Clabir also said it planned to sell its 12.3 percent position in ARC to Sequa Corp. of New York for about $30 a share, or about $33.9 million, close to what Clabir paid for it.

While ARC said it was pleased and believed Sequa was only an investor, O'Leary said she could not rule out a possible takeover move by Sequa, a manufacturing and transportation company. O'Leary said she thought the most recent price decline would make some investors bullish on ARC stock because it is selling for only 10 times her estimate of $2.55 a share for 1988 earnings.

The takeover efforts aimed at Washington Homes of Waldorf, Md., continue on their meandering course.

The latest news is that William J. Harnett, its chairman, and Lawrence M. Breneman, the president, have declared that they don't think either the Sonny DeCesaris Group of Clinton, Md., or the Preference Homes Group of Landover can handle the deal.

"Based on the information received to date, the company has serious doubts about the ability of either SDS {DeCesaris} or Preference to consummate their proposals," the Washington Homes executives said. They added that they have been contacted by other potential suitors, but they didn't identify them.

Meanwhile, DeCesaris has complained that Washington Homes management won't meet with it.