Tonight Dan Rather begins his sixth year as anchor of "The CBS Evening News." Just before he took over the program from an illustrious predecessor in 1981, a West Coast firm was selling T-shirts that asked despairingly, "Oh my God, what are we going to do without Walter Cronkite?"

We managed. And so did the "Evening News." It was a daily newspaper of the air five years ago; it is a daily news magazine of the air now. The most dramatic changes have been wrought by space-age hardware and would have occurred even if Cronkite had stayed on; not only can immediate reports be brought in from virtually any dot on the globe, but the broadcast can be uprooted at short notice and itself originate from a nearly limitless assortment of foreign locations.

Last week Rather was asked how many more years he would like to be anchor and managing editor of "The CBS Evening News." He said, "I hope I can do it one more week." More seriously, Rather, 54, said he couldn't foresee a day of parting with CBS. "As Cortez burned his boats in Mexico, I burned mine at CBS some time ago. This is the place for me as long as they'll have me."

After an early and very uncertain shakedown period -- at first, Rather anchored the "Evening News" sitting awkwardly on the desk, not behind it -- he eliminated or converted the Cronkite holdovers and made the program entirely his own. If Cronkite had the easy authority of a crooner, Rather has the punchier bite of a belter. We have essentially gone from Bing Crosby to Frank Sinatra as anchor of the "Evening News."

We have also gone from Great Uncle Walter to Good Neighbor Dan.

But the program has changed in many ways beyond the mood-setting personalities of its anchors. Lane Venardos, 42, who was Washington senior producer on the Cronkite show, is executive producer of the "Evening News" now. He is in an ideal position to compare the two versions.

"The 'Evening News' was much more a wire service broadcast then," Venardos said yesterday. "Walter was a former wire service reporter. Every day almost we would do stories pegged to some wire service lead. I remember one night we had 13 pieces on the air and all of them were from Washington.

"It was, in a sense, a sausage factory. An in-house version of C-Span. We certainly gave a viewer the important news. But what we didn't provide was what it might mean to him or her, sitting out there in Boise or Roanoke. Now we try to give viewers more they can use, especially when it comes to making decisions about elected officials, tax policies, defense, medical matters, what have you."

In terms of style as well as substance, the "Evening News" is now radically different, just as both ABC's and NBC's nightly newscasts have made quantum technological leaps from then to now. Comparing one Cronkite newscast with one Rather newscast offers innumerable illustrative examples -- in this case, rather arbitrarily, the Cronkite "News" of Jan. 7, 1975, and the Rather "News" of Jan 7, 1985, Rather's 1,001st show.

The Cronkite program that night was dominated by "official" news, much of it from Washington -- congressional reaction to President Gerald Ford's new economic plan, for instance, even though the reaction wasn't very interesting; a report from Detroit on an "auto sales slump" in which business leaders, not auto workers, were heard from; Eric Sevareid reciting a static commentary while sitting motionless and poker-faced in the studio, and so on.

Cronkite simply read the closing piece, which began, "There's a poignant little story out of England tonight." Prince Charles, 26 and unmarried, was saying he was "lonely," Cronkite reported, noting there was, alas, "no prospective bride" on his horizon. Then Cronkite said, "And that's the way it is, Tuesday, Jan. 7, 1975. This is Walter Cronkite, CBS News. Good night."

Illustration on some stories was limited to a large slide projected behind, and sometimes engulfing, the anchor. Virtually every report was on flat film and not bright tape, the opposite of now. Camera work was rigidly correct but almost never imaginative. Nor was there anything to compare with the computer-generated graphics common today. "You see these old shows, with Walter a little dot in the corner and the rest of the screen a map of Gabon, and you wonder precisely what thought processes came up with that format," Venardos notes.

Ten years later to the day and "The CBS Evening News" was a global, rapid-fire pyrotechnical display. Rather anchored the newscast outdoors in Geneva, where the broadcast had relocated for the duration of the January summit conference there. Much of this news was bound to be "official," too, but the broadcast included an ambitious cross-cut report by Bruce Morton in Philadelphia and Mike Phillips in the Soviet Union profiling similarities and contrasts between the American and the Russian people.

Later, for a segment on the feasibility of "Star Wars" by correspondent David Martin, seven different arms experts, none of them official government spokesmen, appeared in pithy sound bites within the space of roughly two minutes. Individual pieces are not dramatically shorter than they were in Cronkite's day, but sound bites, those tiny fragments from interview subjects, now can be much, much shorter. One sentence and you're history on the new "Evening News."

The trend toward longer pieces, evident on all three network newscasts, is a result of increased news competition from local stations, many of which now have their own globe-hopping capabilities and satellite access. All three networks are scheduling more features and special reports. As they do, each network's newscast becomes more distinctive, less predictable; once again, it's that whole new ball game people are always talking about. There's also going to be more dispatching of anchors to remote locales, especially Rather, who says he longs for opportunities to do "real reporting," not just sedentary anchoring.

Are these the dynamics of journalism or of showmanship? Is the desire to keep the public entertained superseding the mandate to keep the public informed? Venardos says, "Last week we did a five-part series on children of the poor. You can bet that no one came around and said, 'Well, here's a sure-fire ratings booster: five pieces on disadvantaged kids.' The issue of whether something's going to be popular never comes up."

Venardos thinks his proudest moment as executive producer (he took over from Howard Stringer, now a CBS News executive, in early 1984) came when Rather flew to Mexico City for extensive coverage of the tragic 1985 earthquake there. Rather spent a weekend in preparation after arriving. Then, on the Sept. 23 broadcast, he anchored live from the front of a demolished building and on tape interviewed the president of Mexico, the U.S. ambassador and weeping women at a makeshift morgue. He was also seen surveying the devastated city from a helicopter.

"It was a watershed for the 'Evening News,' " Venardos says. "It proved our ability to take, on almost no notice, the broadcast out into the field and make it a mini-documentary without sacrificing any of the news of the day."

Rather says, looking back, "When our competitors saw our reports, they didn't know what to do."

After five years on the job, Rather remains as keenly competitive as ever. Venardos says he thinks he can only take the executive producer pressure cooker for one more year; Rather sets no limits. On the air, his competitiveness and zeal translate into a sit-up-and-take-notice urgency that makes one tolerant of his fondness for tongue-twisting-hyphenated adjectives. Some people may think him corny, as corny as Cronkite in August. Tears do come to his eyes at dramatic or harrowing moments. But those who know him find this dauntingly genuine.

Rather's news may appear to be much more the anchor's show than Cronkite's was, but in fact it is not, at least in terms of anchor air time. And Rather is quick to spread credit for the program around. He is lucky to be backed up by the best correspondents, producers and writers in network news.

The ratings race for the "Evening News" is tighter now than it was in Cronkite's day. Recently, "NBC Nightly News With Tom Brokaw" came within a whisker of equaling the program's showing. Rather feigns indifference. Venardos says, "I was concerned. Especially so since our broadcasts that week were all good, no clunkers, and theirs were not anything special."

Rather is too devout a competitor not to be a little worried. After all, given his mania for winning, it probably isn't enough for him that he be considered the best anchor on the air. Dan Rather wants to be the best anchor of all time. All time will tell.