There was a time when the sight of a letter from Katharine Hepburn in my mailbox would have sent the heartstrings a-stirring.

Now Miss Hepburn writes me regularly, and I blush to say I haven't written her back even once, although she is such an anxious correspondent that she has volunteered to pay my postage if I do.

But please don't judge me too harshly. It's not that my regard for Miss Hepburn has diminished. It's not that I'm piqued about her choice of subject matter upon finally resolving to break the long, painful silence between us. A woman can't be expected to express all the thoughts she has in her heart at once. She must begin somewhere. And while I might wish she had begun with some subject other than "reproductive freedom," this is clearly a matter of concern to her; in her words, it is "a basic, personal issue" on which "your help is vital." She is offering me "an unique opportunity to become part of this historic debate."

No, merciful reader, the reason I have not yet taken up my pen to reply to Miss Hepburn is that I'm simply too important a person these days to answer every letter from a movie star. My mailbox, once a metallic wasteland where any arrival was cause for excitement, has become a teeming hovel of correspondence from actors, bank presidents, professors, scientists and other weighty personages. Perhaps your mailbox has suffered a similar population explosion, and I can speak to you as one harried addressee to another.

From Charlton Heston, I have received a special reminder (in a plain brown envelope bearing only his name and a Hollywood address) that my subscription to "American Film" stands at the very brink of expiration. "Please send us your renewal instructions immediately," he writes.

Charles T. Manatt, chairman of the Democratic National Committee, has invited me to lend a hand in helping a select group of conservative Republicans find careers outside the United States Senate. "We don't have a minute to lose," he writes.

William V. White has taken time out from his duties as senior vice president of the National Bank of Washington to attempt to rescue me from a state of blinding financial confusion. "In recent weeks you have been virtually buried under a blizzard of information about money market investing," he tells me. "Whom to believe? What's best for you . . .?"

And Beatrice Bayley, the head of Beatrice Bayley Inc. in Sterling, Pa., worries that I may feel alienated from my ancestors. Two months ago she wrote me to announce that:

"THE LARDNER FAMILY HERITAGE BOOK is being published on Mar. 30 . . . I have spent thousands of dollars and months of work to research through 70 million families and I have located almost every LARDNER FAMILY in these United States . . . Because this guide is rare, THERE WILL BE ONLY ONE EDITION: It is limited to one book per household . . . Orders posted after this date must be declined."

What with one thing and another, I confess that I failed to make Bayley's deadline. But by a stroke of great good fortune, her publication schedule appears to have slipped backward. In any case, she wrote me again a month later to inform me:

"THE LARDNER FAMILY HERITAGE BOOK is being published on May 18 . . . THERE WILL BE ONLY ONE EDITION: It is limited to one book per household . . . Orders posted after this date must be declined."

While I'm not sure just how Beatrice Bayley came to make the Lardner family tree her life's work, it was very gracious of her to give me an early opportunity to obtain her book at the extraordinary one-time-only price of $27.85. I'm less clear about why my household has been singled out by another busy executive, Kalman Finkel, attorney-in-charge of the civil division of the Legal Aid Society of New York.

He begins his letter "Dear Contributor," which is confusing, since the Legal Aid Society has never, to my knowledge, received any funds from the Lardner treasury. But perhaps the word "contributor" is to be understood here in the broad sense--i.e., someone who has contributed something, sometime, somewhere.

At any rate, it turns out that attorney Finkel is after my advice on an internal management question of the utmost delicacy.

"Please help me decide which one of these people needing legal services to turn away," he writes. Then he describes three cases:

* "Jean-Claude M. lives in Brooklyn and works for less than the minimum wage because he is still learning to speak English. Now he finds that the refrigerator he bought for his family a month ago is about to be repossessed because he didn't understand the conditions of payment."

* "Elizabeth K. has lived in the same apartment since she retired from her clerical job 15 years ago. Recently her Social Security check stopped coming and she didn't understand why. Nor does she know how she will obtain the money needed for food and shelter."

* "Curtis G. lives in Harlem with his wife and three small children. The small firm where he worked has closed down and he spends most of his time looking for a job. His family spends their time in an icy-cold apartment. There has been no heat or hot water for the last month and Curtis can't even find out who or where his landlord is."

As a civic-minded person, I would of course be delighted to help the Legal Aid Society resolve its dilemma. But if I did, I'd simply have to turn away someone else's letter, and whose should it be?

Please help me decide.

Should I ignore this poignant note from Allen E. Smith, president of the Defenders of Wildlife? "Picture this," he writes. "A young bobcat shyly approaches a strange hidden object. Curious as any household feline, the bob kitten sniffs, then probes playfully with one small paw. Suddenly, the heavy steel jaws of a metal trap snap shut! italics are Smith's . . . Join us in protecting this magnificent American wild cat and other precious wildlife before it's too late."

Or should I discard Archibald Cox's "special message" on the dangers of nuclear war. "Through the dark days of Watergate and Vietnam, effective citizen action again proved its critical role in our democracy," Prof. Cox recalls, in a note dated "Tuesday morning" and typed on a fragment of yellow legal paper that must have been the nearest thing to hand when he was struck by these awesome thoughts, and the need to share them with me. "As tragic as those events were to us as a nation," he continues, "they were nothing compared to what will happen when the next nuclear bomb is used."

Not all my letters deal with tragedy and injustice on such a global scale. But you would be surprised at how many, even when they come from institutions widely assumed to be interested in the accumulation of profit, turn out to be motivated by deep fellow-feeling and concern for the welfare of humanity.

C&P Telephone, for example, is worried about the plight of Janet Roberts, who works long hours at a shelter for battered women and spends so little time at home that she makes a mere seven phone calls a week. "Yet she pays the same amount of money as someone making 100 or 200 local calls," I am told, because of the D.C. Public Service Commission's refusal to permit more flexible phone rates.

And a company called "The Sharper Image" has sent me an entire catalogue of designer telephones, because it wants to be sure its competitors (C&P, for one) don't exploit me. "Our reputation with you is more important than any profit," writes company president Richard Thalheimer (who signs his note, simply, "Richard").

It is flattering to be the recipient of so much mail from so many influential people on such a variety of urgent questions. (Perhaps the only issue I haven't received any mail about lately is the worldwide paper shortage, which has grown so desperate, I understand, that the average Haitian consumes less than two pounds of paper a year--less than a Sunday newspaper's worth in our neck of the woods.)

It is flattering, but it is also taxing. From time to time, I yearn for the days when a letter was a rare event and there was time to respond to all.

But there's no turning back on the road to celebrity. So as befits my rising status in life, I have decided to compose a form letter for general use.

"Dear Friend," it will say. "Thank you for your recent note on an issue of pressing concern. I have read it carefully and will be bearing your views in mind in the days and weeks to come."

It sounds impersonal, I realize. Katharine Hepburn will undoubtedly be hurt when she looks out the window, sees the postman coming, bounds to her mailbox and eagerly tears open the envelope bearing my name. But she is a woman who knows what it's like to receive more mail than a person can handle, so once she gets over the disappointment, I hope she'll understand.