When convicted spy Arthur James Walker, saddled with debts from a failed radio repair business, decided to turn to espionage for profit, his brother, accused spy John Anthony Walker Jr., gave him a run-down of Soviet priorities, Arthur Walker later recalled.

"The things apparently they were most interested in was code material, crypto as it's called, any kind of top-secret or intelligence-type information," he testified before a federal grand jury in Baltimore.

Arthur Walker apparently failed to produce any such tidbits. "I ain't got no crypto or even know what it looks like anymore," the retired Navy lieutenant commander recalled telling his younger brother.

But information about secret Navy codes and communications techniques is precisely what Jerry Alfred Whitworth, the West Coast partner in the alleged spy ring, is accused of passing to John Walker, his Navy buddy and sailing companion.

Last week, testimony at Arthur Walker's trial and the new, more comprehensive indictment handed up against Whitworth underscored the sensitivity of the information that Whitworth, a retired senior chief radioman, allegedly passed to the Soviet Union.

Whitworth, 45, who held a "Top Secret/Crypto" clearance during at least part of his Navy career, had originally been indicted on one count of conspiring with John Walker to commit espionage. Last week's indictment, as well as charging Whitworth with failing to pay income taxes on some of the $332,000 he allegedly received from John Walker, added new substantive counts of copying, unauthorized possession of, and passing to the Soviets classified information.

The most serious of the new counts, according to military experts, charges that Whitworth in 1980 gave Walker information about the Navy's then-new Remote Information Exchange (RIX) system, a computerized system for transmitting messages to and from Navy installations worldwide.

The indictment charges that Whitworth provided "sketches, photographs, plans, documents, writings and notes" detailing RIX and its "impact on" AUTODIN, the Defense Department's primary communications system, used for transmitting messages via satellite and telephone to military bases and allied governments worldwide.

The indictment does not provide further details about the alleged security breaches.

But such information could have given the Soviets state-of-the-art knowledge about U.S. military communications, enabling them to adapt the techniques for themselves and to narrow the U.S. technological lead, according to military intelligence experts and Navy officials.

"The danger is, it validates to the Soviets how our system is set up," said a former senior Navy intelligence officer. "It gives them another piece of the intelligence puzzle."

Echoing Navy officials' assessment of the overall damage caused by the alleged spy ring, he said of the alleged transfer of information about RIX: "It's serious and it's dangerous, but it's not a catastrophe."

RIX, operated through computer terminals located at naval air stations and Navy supply depots and on ships, sends messages, generally in coded form, to a larger, regional computer system for naval communications. For circulation to other military services, the messages are transmitted through AUTODIN, short for automatic digital network.

Simply by learning the volume and speed of information transmitted through RIX, and through other intelligence-gathering equipment figuring out the destination of the messages, the Soviets would be able to better determine U.S. military intentions and readiness in various locations, military experts said.

If a large number of messages were headed to and from the Middle East, for example, the Soviets would be alerted to concentrate their intelligence gathering on that part of the world. "It gives them a communications roadmap that shows vital intersections," said a Navy official. "If you suddenly see an increase in traffic from point A to point B, you know there's something going on at point B."

Technical details about the workings of RIX and AUTODIN alone would not have enabled the Soviets to read coded military communications, Navy officials said.

However, Whitworth is also accused of giving John Walker "key cards" and "key lists" used in the coding, or "encryption," of classified information.

According to FBI records of its interviews with Arthur Walker unsealed last week, John Walker told his brother that "people can make up to five or six thousand dollars a month" for providing such information. "The best? Crypto. Especially keycards," Arthur Walker reported his brother saying.

The chief of naval operations, Adm. James D. Watkins, has said the Navy "assumes" the Soviets broke codes designed to scramble messages transmitted throughout the fleet by both teletype and telephone, and the other military services have established teams to assess the damage that may have been done to their communications.

In contrast to the information allegedly provided by Whitworth, the two documents that Arthur Walker was convicted last week of giving his brother appear to be of far less significance.

Walker, who was convicted of all seven counts of espionage with which he was charged, admitted giving his brother classified material from VSE Corp., a Chesapeake, Va., defense contractor where he worked as an engineer.

Both documents were marked "confidential," the lowest category of classified information. One was a "damage control book" on the USS Blue Ridge, a Navy training manual, containing a wealth of engineering details and specifications, on how to handle various kinds of damage and breakdowns aboard ship. The second was an extract of "casualty reports" summarizing four years of breakdowns and equipment failures about Navy amphibious helicopter assault ships.

Capt. Edward Sheafer, a senior intelligence officer on the staff of the Navy's Atlantic Command in Norfolk, termed the damage control book "a Bible for sabotage" of "tremendous benefit to a potential enemy" because it would give information about ways in which the ship is vulnerable to attack.

As to the casualty reports, Sheafer testified, "I'd give my eye teeth" for that kind of information about the Soviet navy.

But another assessment of the value of the documents came from John Walker himself, his brother said.

According to FBI reports of its interviews with Arthur Walker, John Walker "told him that what he had gotten so far was no good." If that was the best Arthur Walker could do, he recalled his brother telling him, "Don't waste your time or take the risk or make me take the risk of carrying it around."