You're short lunch money the day before payday.

Do you go to your banker and ask for a loan? More than likely you turn to a co-worker and borrow $5 for a few days. To formalize the transaction, you could give a personal IOU.

Corporate treasurers are doing the same thing, in increasing numbers.

Instead of turning to their bankers for a loan, they borrow directly from other companies for 30, 60 or 90 days (even as long as 270 days) and give their creditors an unsecured promissory note called commercial paper.

It is nothing more than the corporate version of an IOU. It is also much cheaper than a bank loan.

While eyes in Washington and New York have focused on the growth of the money supply and bank loans, today the fastest growing source of corporate cash is the commercial paper market.

Bank loans to businesses grew about 15 percent between February 1978 and February 1979. During the same period, commercial paper outstanding jumped nearly 33 percent.

In February 1978 there was $65.9 billion of commercial paper outstanding. Last February there was $87.4 billion within the last two weeks the amount climbed above $90 billion.

"It's a very rapidly growing market," according to Chase Manhattan executive vice presidnet John Hooper."You've got to be concerned about that."

If history is a guide, every mushrooming market contracts for some reason. In financial markets that reason generally is skyrocketing interest rates and a fear of the future that drives investors from higher risk investments, the "flight to quality" as it is called.

Commercial paper is not new. And the commercial paper market has had its problems in the past, most recently when the Penn Central went bankrupt in 1970 and when many real estate investment trusts faced financial crunches in 1974.

In both those cases, banks were on the hook. If today's burgeoning commercial paper market comes a cropper, banks would again take it on the chin.

Although the commercial paper market takes lucrative business away from banks, the market could not exist if issuers of commercial paper-finance companies, industrial companies, utilities and even bank holding companies themselves - could not arrange back-up lines of credit with their banks.

The same competition that forces banks to shave their interest rates, compels them to furnish the back-up lines of credit to issuers of commercial paper. "Don't ask me why," said Donald C. Miller, vice chairman of Continental Illinois Bank and Trust. "We don't get paid adequately for the service."

The hitch with commercial paper is its short maturity. Most of these corporate IOUs are due less than 60 days after they are sold.

If a company cannot pay off its maturing paper and cannot sell another issue, the company must turn to its bank and draw on its line of credit.

That happened - in a much smaller commercial paper market - in 1970, when a bankrupt Penn Central defaulted on $82 million of commercial paper and edgy investors were reluctant to purchase new issues of paper from smaller, less well known companies.,

Real estate investment trusts caused similar problems in 1974. At the start of that year, REITs had $1.8 billion of outstanding paper. At the end of the year, they had $175 million. The investment trusts came up with most of the difference by borrowing from their banks.

Even in the worst of times, no one would expect the $90 billion commercial paper market to collapse. For the most part, the issuers of these corporate IOUs are the Tiffany's of the borrowing set.

General Motors Acceptance Corp., which finances many car purchases by GM customers, is the rock of the market, with about $9 billion of paper outstanding. For accounts for $5.5 billion of it, Sears $4 billion and General Electric $2.5 billion.

Furthermore, according to George Van Cleave-in charge of commercial paper dealing for Goldman, Sachs-there are three companies that carefully rate issuers of commercial paper and set better standards than a decade ago.

Still, the lure of commercial paper is bringing more and smaller companies into the market every month. Recently, savings and loan associations were given the right to issue commercial paper by the Federal Home Loan Bank Board. (Banks cannot issue paper, but the holding companies that own them can and do. Citicorp, for example, sells $100 million a week. It explains why banks have a love-hate relationship with commercial paper).

"As long as a company can sell its paper for 9 3/4 percent and would have to pay a bank 11 3/4 percent (the prime rate at most banks today), it's going to want to sell commercial paper," said Chase's Hooper.

Even when the costs of selling paper through a dealer (or, as is the case with a big issuer like GMAC, maintaining its own sales staff) and paying for a backup line of credit are figured in, a company will save a percentage point or more by selling commercial paper rather than borrowing from a bank.

Today the market looks stable, according to Robert Volland, director of financial sales for Commercial Credit Corp., one of the oldest and largest issuers of commercial paper (about $1.1 billion outstanding).

"I don't know what I'll say two years from now," Volland said.

"It's true that when interest rates rise or there is a credit crisis, it is then that the lesser known companies have their problems."

The commercial paper market will continued to be alluring especially if an inflation-weary Federal Reserve continues to increase interest rates.

While 85 to 90 percent of all commercial paper has the top grade from rating services (Moody's, Standard & Poor's or Fitch), severe problems in refinancing by that bottom 10 to 15 percent could put severe strains on a banking system that is also being squeezed by the Federal Reserve.

"I always remember," said one former stock broker, "that we always said one of the big advantages that real estate investment trusts had was that they could issue commercial paper. It didn't help them at all.

"We're now hearing the same thing about savings and loan associations."

As Commercial Credit's Volland cautions, check back in two years. CAPTION: Chart, no caption, The Washington Post