District of Columbia and Maryland health officials who are monitoring radiation levels said yesterday they were encouraged by low levels recorded following the accident at the nuclear plant near Harrisburg, Pa.

"If everything goes the way it is now, I think we're in good shape," said Ron Nelson, administrator of Maryland's community health program.

For the first time since the nuclear accident on Wednesday, however, minuscule amounts of an inert radioactive gas known as xenon 133 registered in air samples taken in Cecil and Harford counties, Nelson said. But the amount were "so low, we can't assign a number to it on our computer," he said.

Officials were "very encouraged" by the absence of another radioactive gas-iodine 131-which Nelson said "falls on the ground and could easily get into the milk supply" and attack human thyroids. By contrast, xenon 133 "dissipates rapidly" and "is not stored in the body," Nelson said.

Maryland health officials found no radiation "above background levels" in two water samples taken from the Susquehanna River Sunday night and in six milk samples taken early yesterday from dairy farms in northeastern Maryland and nearby Pennsylvania.

"We're going to pick up river waters tonight and milk samples again tomorrow morning and air at noontime," Nelson said.

In the District, results of milk and water samples were not yet in-they had been sent to Atlanta for analysis-but air readings were described as "nothing unusual" at .31 picocuries per cubic meter. A picocurie is one-trillionth of cuire, a basic radiation measure. The highest such reading ever recorded here was 1.7 picocuries per cubic meter, monitored after a Chinese atomic test in 1977.

The weather also was keeping any radioactive air away from the Washington area, with winds coming generally from the south and southwest.