The Internal Revenue Service is about to take a hard look at the millions of Americans who rent out second homes or vacation properties for part of the year.

During 1980 and 1981, IRS will be particularly interested in what these owners claim in the way of deductions on their properties.

The IRS has just approved draft regulations for the "vacation home" changes in the tax law passed by Congress in 1976. The regulations haven't been issued publicly by the Treasury, but should be out in the near future.

Although three years late, formal issuance and enforcement of the rules could jolt large numbers of home owners who had never heard about the 1976 law changes in the first place.

The regulations also could prove a shock to people who don't own what they consider "vacation" real estate at all, but rent out condominiums in the city, homes in the suburbs or even houseboats tied up at marinas.

The key provisions in the law relate to "personal" usage and deductions. If you occupy any residential property (or boat) you own for more than 14 days a year, or for more than 10 percent of the total number of days that you rent it out during the course of a year, you've gone over the IRS's tripwire.

Once you've gone over that line you are prohibited at tax time from writing off as deductible expenses any more than you received as rental income from the property. In plain terms, you can't take a tax "loss" on your real estate -- a practice that once made ownership of vacation and second homes the most pleasurable tax shelter available in the United States.

Once you've tripped the wire, you cannot add up depreciation, travel, repairs, mortgage interst, property taxes and the myriad other expenses that pop up at the beach or at the lake, and expect Uncle Sam to help you pay for them as generously as he once did.

Fair enough, you might say. After all, if people are making personal use of a place, rather than running the property as a true rental business, then they shouldn't get a federally subsidized tax shelter.

But here's the rub. The definition of "personal" use -- to be counted by IRS against your 14-day tripwire -- could surprise you.

For example, any rental of your property to an immediate relative, such as parents, brother, sister, son, daughter, grandparents or other "lineal" members of your family, will constitute personal use.

It won't matter that you charge your brother and his family the full, going market rate when he rents your ski condo or beach cottage. It won't matter that you were 3,000 miles away, and never visited the place during the entire time your brother's family was using it. As long as he's there, it counts in IRS's book as if you were there 24 hours a day.

Another potential trouble spot for you under the rules could come from the rental rates you charge. If IRS audits your tax return, and decides that you charged a tenant less than a "fair" rent, that tenant's period of residence will be counted as personal use by you.

Say, for example, that you often charge friends or acquaintances $200 a week to rent your place, instead of the $300 you charge everyone else. Think twice in the future about such acts of kindness under the vacation-home rules. The IRS could tack those weeks onto your personal use column and cut your rental property deductions drastically.

(The "fair" rental issue, by the way, poses potentially serious problems for the owners of any residential real estate -- in cities, suburbs, or rural areas. If IRS decides that the rent level is too low to be accepted as "fair," it could deny owners their normal depreciation and other deductions, construing the rental as personal concession to the tenant at variance with a normal business relationship. An IRS regional office has already taken this position in an audit of a rental home in Chicago.)

Watch out, too, that you carefully document the time that you spend fixing up your vacation property. Any time you're present at your ski cabin or beach property -- painting the walls, repairing the deck, caulking and cleaning -- but cannot demonstrate that you spent a "normal day's work" in such maintenance activities, the IRS may claim that your real purpose at the property was personal use or enjoyment.

Although the IRS probably won't send many of its agents skulking around vacation areas taking photographs and checking utility meters, keep in mind the agency's potential for giving you a rough time on next year's tax return. An IRS source said his agency plans to select out a number of vacation and rental home owners for audit, and "slap violators hard, enforce the law very strictly, to make an example."