Yesterday's Washington Post told us that during the next five years the cost of local government will shoot up at an almost unbelievable rate.

In this case, "local" means not only Washington, D. C., but all the suburban jurisdictions that surround it.

Any affluent suburbanite who has been enjoying Mayor Marion Barry's budget problems can stop smiling now. The suburbs are about to find themselves in the same stewpot.

Costs are up. Federal aid is down. Voters are in rebellion over rising taxes. County employees are in rebellion over wage scales that do not keep pace with inflation. And the poor, the sick, and aged and the unemployed want more help, not less.

But a report just issued by the Greater Washington Research Center says that even it the area's local governments "slice spending in half" and bring in millions more by raising their sales taxes and property taxes, they won't be able to balance their budgets.

The projection is that in five years the deficit will be $600 million to $3.6 billion. Even if the counties "slam on the brakes" and cut annual spending increases in half, spending will double between now and 1985. The warning that we're heading into deep trouble is loud and clear.

How will we react to it?

Will we begin to look for remedies at once? I doubt it.

I think we will limit ourselves to talk for a few years. Some people may even talk sensibly about the subject, but they won't draw much of an audience or have an appreciable effect.

My guess is that we will take little or no action until the first substantial deficit actually materializes.

Then we'll roll up our sleeves and do what every God-fearing, red-blooded American does when the time for financial belt-tightening is at hand. we'll borrow.

That will make mattes worse, not better. One who borrows must pay interest in addition to all the things he couldn't pay before he borrowed.

By the time we are forced to face up to the difficult decisions we should have made in 1980, or perhaps even decades earlier, we may find that some of our options have evaporated.

We may be so far down the road toi ruin that the only answer to, "How can we get back?" will be, "Sorry, you can't get there from here."

Aren't these lovely thoughts for the start of a new week? No wonder people hate Mondays. POSTSCRIPT

So long as the morning is ruined anyhow, we might as well take note of the weekend's demonstrations in favor of peace and against war.

I found them very instructive because it had never before occurred to me that peace is better than war. And gee whiz, I'll bet there are lots of other people who also never bothered to think through that complicated proposition and form an opinion on it.

All I can say to the demonstrators is: "Golly, I sure am indebted to you for opening my eyes. It really is much better to kill ourselves with overdoses of drugs, or by floating along in our muscle cars at 75 miles an hour, or by eating, drinking and smokingas we please and not letting any stupid over-30 adult order us around.

"Thank you for the demonstration.The worst thing that could happen to you would be for it to succeed.

"Old people like me won't be around long enough to endure a world dominated by the USSR. You would be, and it would serve you right.

"Inasmuch as you are demonstrating your determination not to be prepared to respond to a Soviet thrust, I think you own us a demonstration of how you plan to cope with the inevitable Soviet move to fill this power vacuum.

"If President Carter and the Congress were foolish enough to let you intimidate them, if they were to fail to prepare for a Soviet move into the Mideast oil fields, how would you provide the energy needed to grow or transport the food you eat? How would you manufacture the things your parents buy for you? How much electricity could you generate by burning all your 'No Nukes' signs? In short, dear demonstrators, what in the hell gives you the notion that you have a monopoly on wisdom?"

Every sane person over the age of 10 knows that peace is preferable to war, and that peace is worth working for. Only an egomaniac would assume that he alone has discerned this great truth and has been anointed by the Almighty to preach it to the masses.

Placards bearing solgans like "We won't die for Citibank," "Say No To Racism and Sexism" and "Ratify ERA," all of which were prominent in the "anti-draft demonstration," served only to confuse me.

Are people who back rational causes like equality aware of the damage they do themselves when they crawl into bed with kids who say they're willing to let their country go down the drain rather than register for a possible call to national service?

Good heavens! Is it already too late to get there from here.?