A MAN HAS TO take his boy fishing. It's a rule, at least to those of us who grew up with Norman Rockwell's Saturday Evening Post covers. (A man has to take his daughters fishing too, these days, but that is a new rule and another story.)

No father worthy of the title will neglect his responsibility to wet a line with the lad. There are things a boy must know about a man's estate that he can learn no other way.

Getting dirty, for instance. A boy can splash in mud puddles and roll on the ground and smear himself with peanut butter and jelly, but he suffers from a prepuberty glandular deficiency which makes it hard for him to get really good and stinking dirty.

(Women may wonder why the male of the species is so fond of being grubby. It's because it makes the showers feel so good. Few women have ever had a decent shower, because they always jump the gun. Woman works up a mild sweat, just starting to get greasy around the pores and smell like a human being, and she goes and takes a bath.The odor of a women's locker room has no texture to it; it's too thin and arrid. Never mind how I know.)

A boy seldom can find a dead cat or a live skunk these days, and rassling with a wet dog is a poor substitute. Spending an afternoon on a riverbank among reeking mud, slimy worms, musty crickets and dead fish ranks behind only cesspool cleaning and five days in a deer camp as a way to get genuinely ripe.

And how is a boy expected to learn to cuss properly if he has not been present when his father tries to cast and discovers that the tackle shop wound the new line on his reel backwards?

And killing things. It is in the nature of boys and men to kill things, this having been their function since the race began. But people don't slaughter their own hogs and chop the heads off their own chickens in their own back yards any more, which means a boy must be content to skoosh bugs until he's old enough to hunt. And then he'll have to put u with a lot of hooraw from the parlor predators who choose to ignore where hamburgers come from.

Fishing is the answer. There is as yet no Friends of Fish Inc. lobbying to outlaw the rod and reel. People who call themselves vegetarians eat fish, the rationale being, apparently, that it's all right to spill cold blood. Boy kills a bunch of fish and even his mother is likely to applaud, so long as his father cleans them.

And secrets and mysteries. Television takes today's child into what he thinks is mystery and suspense, but it's always spoiled because the mystery invariably is solved. No one knows what a mystery is until he has tried to puzzle out why fish bite one moment and not the next, and no suspense compares with what the fisherman goes through between the strike and the landing - or the loss. It's the ones that get away that are best, and on TV nothing ever gets away.

It was to help him learn these and other things that I took Mark, 4, out to Fletcher's Boat House on the Potomac above Key Bridge the other day. He learned a lot.

He learned that muddy rocks are very slippery, by watching his father slip and scatter bait and tackle all over the place.

He learned about what you do when the bathroom's too far away.

He learned, from seeing his father moan and such his finger, that fishooks are very sharp and may have to be cut out of a careless person's skin.

He learned how hard it is to count a dozen ducklings as they bob and weave through the water behind their mother, and he learned something he didn't want to know when she came paddling back again an hour later with only eight.

"What happened to them?" he asked.

"Something ate them."

"Did a alligator eat them?"

"No, a fish maybe, or a turtle."

"Oh. They were pretty."

Mark also learned that while a father may be able to pull coins out of your ear and splinters out of your foot, it does not follow that he can pul fish out of a river.

"Where are the fishes?" he wanted to know after a half-hour of tending his cane pole and watching Dad cast a whole series of cunning, expensive and unproductive lures.

"They're out there. They just don't seem to be hungry right now."

"I'm hungry."

"It won't be lunchtime for a long while. You just had breakfast."

"When will the fishes come?"

"I don't know."

"Can I go play?"

"Sure. Be careful."

He spent the next hour romping on the rocks, sppearing occasionally to inquire, "Hey Dad! How many you got now?"

At length the lack of action began to make even the office seem attractive, and I unrigged the rods and went looking for Mark.

He was tending a school of minnows imprisoned in a rain pool. "Hey Dad! Look at this!" he said, holding up a couple of four-inchers.

"Where'd you get those?" I asked.

"In the river!"

"What did you catch them with?"

"In my hands."

"You're kidding."

"Yes I did. See."

He went to the river, dipped his chubby little hand into the opaque water and, after a moment of feeling around, flipped another fish onto the bank. "It's easy," he said.

"Let's go get some lunch," I said.

"Can we come catch some more fishes tomorrow?"

"Ask your mother."