The U.S. Postal Service has added something else to the snow, rain, heat and gloom of night that will not stay its couriers from their appointed rounds.

It is called nuclear war.

And the Internal Revenue Service, true to its reputation, is intending to hound you to the end of the earth for its tax money with a post-nuclear-war tax plan.

Postal planners donned straight faces yesterday and went before a House Post Office subcommittee to outline a 400-page plan for delivering the mail after a holocaust.

What would happen, wondered Rep. Edward J. Markey (D-Mass.), if not many people were left to read and write letters after a nuclear war?

"Those that are left will get their mail," said Ralph H. Jusell, the civil defense coordinator at the Post Office.

Jerry K. Jones, the chief planner, was just as reassuring. "If something is left, we'll be able to respond," he said.

If it sounded ludicrous, it was supposed to. Rep. Mickey Leland (D-Tex.), the subcommittee chairman, said he plainly intended to turn his stately hearing room into a theater of the absurd.

"There was some giggling around the Capitol about the absurdity of this hearing," Leland said. "But we are all affected. And with that I thought I ought to do my part to point out the absurdity of this nuclear madness."

Before Leland and his colleagues finished, they had stamped "dead letter" all over the plan and converted their hearing into a denunciation of the administration's stepped-up civil defense planning.

The postal plan that riled them calls for moving postal operations to remote areas to continue mail handling and providing fallout protection for postal bigwigs and workers.

Under the plan, the Post Office also would pass out emergency change-of-address cards, help censor international mail and register federal workers and enemy aliens. But, alas, it would have to stop handling food stamps, passports and migratory bird stamps.

Markey, a leader of the congressional nuclear weapons freeze movement, wasn't impressed. After he pointed out flaws in the plan -- lack of fuel, ruined buildings, destroyed highways and the like -- Jusell and Jones looked at each other and shrugged their shoulders.

Other witnesses took up where Markey and Leland left off.

Roger Molander, executive director of Ground Zero, retired rear admiral Gene La Rocque of the Center for Defense Information and Dr. Paul Milvy of the Physicians for Social Responsibility raked the postal plan over the coals.

"This whole civil defense scheme is the most egregious waste of the taxpayers' money I have ever witnessed," La Rocque said. "Worse than the waste of money is the fact that the civil defense program serves to create the impression that nuclear war is fightable, winnable and survivable."

Joseph A. Moreland, who helps plan mobilization preparedness at the Federal Emergency Management Agency, didn't take kindly to the public belittling of the survival blueprints.

He told Leland he agreed that it was impossible to contemplate the ruin in a post-nuclear-war America, but no matter. If "the event," as he called it, occurs, he said that FEMA's job is to deal with it through a government-wide plan.

Moreland said that FEMA is moving quickly to draw up plans for President Reagan's new Emergency Mobilization Preparedness Board. An assignment is an assignment, he said, and FEMA is serious about doing it right.

Rep. Robert Garcia (D-N.Y.) listened stoically, and then told Moreland he had no use for the planning. "If the bomb comes, I hope it hits me in the middle of the head," Garcia said. "I don't want to be 10 or 15 miles away . . . . The devastation is going to be horrendous."

Rep. William L. Clay (D-Mo.) told the postal people their idea was "lunatic," and then admonished them not to deliver him any telephone bills after the bomb hits. "You can keep their bills," Clay said.

While the Postal Service feels confident about its plan, there's apparently more doubt at the IRS, where they're trying to work out a post-disaster tax-collection scheme.

A memo circulating at the Treasury Department, prepared by tax analyst Gary Robbins, suggests that a nuclear blowout would destroy a lot of government and private tax records.

And, since the income tax system is based on reliable record-keeping, Robbins wrote, a postwar government in all likelihood would have to scrap the income tax.

The easiest solution would be a national sales tax geared to the amount of money the government needed to make things hunky-dory again. Robbins wrote that a tax of about 20 percent would do the job.

But there's something important to remember, he cautioned. "The tax must be calibrated . . . to permit the increased government spending to be accomplished without inflation."

A Treasury spokesman said the paper was only for planning purposes. But he denied reports that the Feds are considering the use of salt instead of currency after a nuclear war. "That really sounds crazy," he said.