It is Henny-Penny time again. You remember the story: an acorn fell from an oak tree and hit poor Henny-Penny on the head. ''The sky is falling!'' she cried, and she fluttered about the chicken yard scaring the other birds quite out of their wits.

To listen to the lugubrious cackles from the right wing, you would suppose that President Reagan has taken leave of his senses. He is signing an agreement with Mikhail Gorbachev to achieve a reduction -- a very minor reduction -- in nuclear arms. We need a soothing voice to say, hey, girls, this is only an acorn. Cool it.

The most hysterical outburst comes from two gentlemen who identify themselves in a formal statement as ''conservative leaders.'' These leaders are Richard Viguerie and Howard Phillips, both of Washington. They have nominated and elected themselves cochairmen of ''The Anti-Appeasement Alliance.'' The gravamen of their complaint is that the president ''has resurrected the disastrous policy of de'tente.'' He is plunging ''headlong into another Munich.''

''It is with deep regret,'' says Viguerie, ''that we who have supported President Reagan in so many battles during the past 20 years now must begin to publicly separate ourselves from our former leader. We feel alienated, abandoned and rejected . . .''

Human Events, the conservative weekly, worries that Reagan is about ''to give away the store.'' My brother pundit Bill Buckley fears that our policies are serving the Soviet Union to such an extent that eventually we will ''lay down our arms.'' The Wall Street Journal warns that if the pending agreement on intermediate nuclear forces should be ratified, the Soviets ''will not lack for nuclear weapons.'' Assorted Republican presidential candidates, not having read the agreement, are pronouncing themselves dead set against it.

Now, Messrs. Viguerie and Phillips are extremely vocal fellows -- they are extremely everything -- but their notion that they are ''conservative leaders'' is a notion to be regarded with extreme dubiety. They are so far to the right they are not even in the ballpark; like a pair of doleful vultures they roost on a billboard overlooking the bullpen. Their charge that Reagan is engaged in ''appeasement,'' to put the matter mildly, is poppycock. As for the disastrous policy of ''de'tente,'' the noun is defined as an easing of tensions and strained relations between nations. Their view is that the greater the strain and the tighter the tension, the better; but that is not a view that is universally held among conservatives with longer credentials.

Human Events is a world-class worrier; it always worries. Brother Buckley, I think, was just having one of his bilious days; to The Journal's profound observation that after INF the Soviets still would have nuclear weapons, a reasonable response is in order: so would we.

I do not understand this fluttering of the right wing. The Henny-Pennies would have you believe that without intermediate nuclear arms, the West would stand naked before its enemy. What hokum! What absolute rubbish! The sky isn't falling. It's right there.

It is there in nearly 11,000 nuclear warheads in the U.S. strategic arsenal, and these are not cream-puff warheads. On average, each warhead carries the destructive equivalent of 300,000 tons of TNT. A nuclear deterrent remains in the Trident submarines, in a modernized fleet of bombers, in a superbly equipped Navy that nears its goal of 600 combat ships.

What is this talk of the United States ''laying down its arms''? This is twaddle. Does anyone seriously propose that with this agreement, we are on our way toward disbanding 28 Army divisions? Do we cut up the new M1A1 tanks for scrap? In the past few years seven Aegis cruisers have joined the fleet. They are formidable ships of war. The force of attack submarines has grown from 73 in 1980 to nearly 100 today. The Soviet Union has no match for the capacity of our Military Airlift Command.

As opposed to the ''disastrous policy of de'tente,'' let me nominate the catastrophic possibility of all-out nuclear war. Is it sensible to reduce that possibility? It seems so to me. In moving step by cautious step toward relieving the fears of mankind, both Reagan and Gorbachev are acting responsibly. This isn't appeasement. This is common sense. It is a characteristic that Henny-Penny, sad to say, conspicuously lacked.