IF YOU'VE been worrying that the members of Congress might have left town without receiving any Christmas presents, you can relax. They took care of that matter themselves just before they left.

We'll start with a mention of what Santa Claus did not bring members of the House and Senate: a raise. There hasn't been an increase in congressional pay since 1979, and that one was a modest 5.5 percent. Maybe a raise is actually deserved. It certainly would have been a straightforward, aboveboard way for the members to reward themselves for their labors.

But congressional raises are hard to vote for, even in a year when all other federal workers received them. So, instead of this politically risky but visible approach, Congress gave itself two less obvious presents. These personal revenue enhancers were adopted bit by bit in a series of amendments to unrelated bills. But, assembled in the manner of a Christmas toy, they don't look bad.

First, using the precedent that businessmen traveling away from home can deduct their living expenses, members authorized themselves to deduct from income all the expenses of living in Washington. The Joint Committee on Taxation estimates that this tax break will be worth as much as a raise of $10,500 to the average member of Congress. But it could be even more. Included as deductible expenses are rent, food, entertainment, transportation and--here's the biggest break of all--depreciation on their Washington homes.

Anyone who owns real estate in this town knows what a bonanza that could be, especially if the congressman's Washington home is really his main manse, housing his family, and if his residence in his home state is little more than a mail drop. The depreciation figure would be limited if families lived with the member. In a four-person household, for example, only one-quarter of the depreciation figure could be deducted. Next, the new law directs the secretary of the Treasury to set a figure--$75 a day was suggested--that would automatically be deducted without fear of audit. If expenses, including depreciation, were actually more than that, the full deduction could be taken so long as the congressman could prove actual expenses to the Internal Revenue Service. Finally, in a burst of good will and holiday spirit, the legislators changed the law to make its provisions retroactive to Jan. 1, 1981.

The second gift was a revision of a self-imposed earnings limitation that went into effect in 1979. This rule limited the amount of outside income that could be earned by a congressman to 15 percent of his salary, or $9,099.37 a year. The rule did not restrict income from sources other than work, such as dividends, interest or profits from a family business, but it did impose a ceiling on what could be earned in outside activity, such as speechmaking, writing and personal appearances.

Right from the beginning it was apparent that this law pinched. While some members of Congress were never offered a nickel to expound their views to a live audience and others were willing to orate for the sheer joy of hearing themselves, some members had made a substantial amount of money from these moonlighting speeches. In 1978, one senator cleared more than $50,000 over and above his Senate salary on the rubber chicken circuit. One thousand dollars is a small fee for a senator's speech, and $10,000 is not unheard of as an inducement to get a committee chairman to address a room full of corporate executives.

Many members had come to count on this opportunity for extra income, and they chafed under the self-imposed restriction. The senators were the first to act. First, they postponed the implementation of the law; then they raised the outside earnings ceiling to $25,000; then, in the closing days of this session, they eliminated the ceiling entirely. The House, more modestly, simply doubled the limit to $18,198.75.

This all seems to be a terribly complicated and unfair way of compensating members of Congress in the absence of a pay raise. The rewards are so uneven. It's far better, for example, to be a powerful Senate committee chairman commanding large fees for speeches and living with one's family in a $300,000 house in Wesley Heights than to be freshman congressman from New Jersey commuting home every night to see one's family and constituents. But as the midnight oil burned and the clock ran out, Congress decided that two flawed gifts were better than nothing at all. So it took the money and ran