If you use a car or your personal computer in your business, or if you entertain customers, hand out gifts or travel in connection with your work, new and stricter rules on the associated record-keeping went into effect last week.

The Tax Reform Act of 1984 now requires "adequate contemporaneous records" to substantiate these business expenses. The "contemporaneous" requirement means necessary information must be entered in some kind of log or diary at the time the expense is incurred.

(It appears that you will be able to reconstruct the records after the fact if the original records are lost due to circumstances beyond your control such as a fire.)

How about "adequate"? At a minimum, your records probably will have to show all of the following items that apply: the time and place of an event; the amount of an expense; the amount of use (i.e., mileage for a car, time for a computer); the business purpose, and the name and business relationship of each person involved.

Commercial stationers and most other stores with stationery departments now have business expense logs for sale at relatively low cost. These are handy, but not required. You may keep your records in a daybook or diary or in blank memo book if you prefer -- as long as you record all the necessary information at the time of each event or soon after.

According to the Tax Reform Act, any underpayment of tax as a result of failure to maintain the required records will be considered negligence. In addition to losing the deduction, you leave yourself open to a neligence penalty.

When you file your 1985 tax return next year, if you claim a deduction for any of these business expenses, you will have to certify that you have maintained the specified records.

If you use a professional tax preparer, he or she will be required to obtain a written confirmation that you have the necessary records. If you don't provide such a statement, the preparer may not sign the return. And if the tax preparer does not comply with these rules, he or she will be subject to a $25 penalty for each failure.

So do yourself a favor: If you are engaged in any of these business-related activities, and you haven't already started to keep the statutory records, get yourself some kind of record book right now and start keeping "adequate contemporaneous records."

Q: We are concerned about the effect a flat tax will have on the value of long-time municipal bonds. What would be the effect on an investor now in the 36 percent tax bracket and holding municipal bonds paying around 10 percent?

A: Passage of a "flat tax" -- an income tax structure in which everyone pays at the same rate or a modified flat tax in which there are many fewer rate brackets -- will have no effect at all on the interest you will receive. You will still get that 10 percent.

The problem is in "equivalent yield." Assuming, for example, that such a new tax structure results in your being in a 25 percent bracket instead of your present 36 percent bracket, the value of that 10 percent yield drops.

In the 36 percent bracket, you would have to get 15.625 percent in a taxable return to end up with 10 percent net; in a 25 percent bracket, that same tax-free 10 percent is only equivalent to a 13.333 percent taxable return on investment.

That lower equivalent yield acts to reduce the value of the municipal bond. If someone were looking to buy the bond, they would only be willing to pay a lower price than before in order to boost the actual return and in turn the equivalent yield.

If you hold the bond to maturity, you will still get the face amount, and you will continue to get the same 10 percent payout.