SAN FRANCISCO - What was supposed to be a Neil Young guest appearance at a Pearl Jam concert turned into quite the opposite here yesterday after an ailing Eddie Vedder was forced to leave the stage just 25 minutes into the show.

"I had a really bad experience last night," Pearl Jam's frontman told the sold-out crowd of 50,000 that packed the Polo Field in Golden Gate Park.

"I went to the emergency room at 3 in the morning and I was puking my guts out, so I think this is it for me," he said without identifying his affliction.

"Lucky for you, Neil Young's here and he's going to take over."

With that, Vedder walked offstage and the biggest headlining show of the band's career came to abrupt end after seven songs.

Twenty minutes later, Young who is about to release a new album, Mirror Ball that features Pearl Jam, officially took over a gig he was only supposed to drop in on.

Backed by the rest of Pearl Jam, he previewed seven of the new songs and generously tossed in lengthy classics such as Cortese the Killer, and Down by the River.

The restless crowd, however, had clearly come to see Pearl Jam. Many even booed at the end when the group's bassist Jeff Ament came out and apologized for Vedder's illness.

As an angry Young snapped before launching into an encore of, ironically, Peace and Love, "I just want you all to know that the last time I saw Eddie (backstage), he was lying on his face. I know he gave it all he had."

Meanwhile, yesterday afternoon's event was also seen as a crucial test for ETM, the new ticketing agency Pearl Jam has been using while waging its war against Ticketmaster, and which the group says it will also use if it tours Canada this fall.

The verdict? ETM passed with flying colors despite dire warnings that the system couldn't handle 50,000 tickets - especially since each of those tickets had to be passed through an electronic scanner at the door. Everyone was moved in quickly and efficiently.

The Haight-Ashbury Free Clinic was on hand for any emergencies, and the security personnel were probably the most unheavy-handed bunch ever to work a major concert.

In fact the most severe warning they issued all day was for the people outside to "chug-alug your beer now, because you can't take it inside."

To say nothing of the ticket price, an almost unheard of $23.95. And that includes taxes and service charges.

If this is indeed Pearl Jam's idea of the brave new world of rock concerts, by all means bring it on.

SUN RATING: 4 OUT OF 5