The premiere of the CW’s “90210″ — arguably the most crucial series of the fall season — became the highest-rated scripted debut in the network’s two-year history.

Tuesday’s two-hour premiere was seen by 4.9 million viewers and garnered a 2.6 rating among adults 18-40. That surpasses the CW’s previous scripted record in the adult demo by 18%, set by a premiere of the long-running cult favorite “Smallville” (2.2). The “90210″ number also ties the network’s premiere record once reached by reality hit “America’s Next Top Model.” The CW enjoyed a rare full-night victory, besting all broadcast competitors to win the evening in the adult demo. Among the network’s women 18-34 target, “90210″ was the CW’s highest-rated scripted telecast ever (4.3/12).

“I was really thrilled, I was relieved, I was excited,” said CW entertainment president Dawn Ostoff when asked her reaction to the numbers. “I knew this would appeal to a broader audience than our other shows. We are really happy with the Nielsen numbers, I really think this show is going to soak into the zeitgeist.”

TV critics were braced for disaster after CW refused to send out screener copies of the show (the debut was actually reworked until nearly the last minute). But most critics were generally pleased with the result, calling the show a serviceable-if-tame update of the 1990s original.

The “90210″ returns are considered to have outsized importance. No network has more riding on a single show than the CW has on “90210.” If the CW does not improve, industry insiders have whispered this summer, corporate parents Warner Bros. and CBS Corp. might fold the network. And of all the CW’s new shows, only the teen soap sequel was considered to have a real shot of significantly moving the needle. To paraphrase the network’s “90210″ slogan, the CW is living by the code.

“It needs to be something that is surprising,” a company insider said last week of the “90210″ rating. “An ‘I didn’t know the CW could do that kind of number’ number.”

By setting a new scripted record, last night’s tally could fit the bill. Combined with “Gossip Girl” showing some growth on Monday, “90210″ should give CW some breathing room if the show can continue to carry a fair amount of its number into next week. Now that’s a big If, since last night doubtless contained a significant amount of one-time curiosity tune-in.

“I think it’s probably excessive to pin the survival of a network on a single show, but unquestionably the relaunch of this well-known franchise is going to be inordinately important in bringing some heat back to the CW, and forestalling further erosion of their audience,” TV historian Tim Brooks said. “Youth-oriented networks, more than anyone, need heat.”

The “90210″ numbers also improved over the course of the show’s two hours among all key demographics, which is a hopeful sign that audiences remained engaged in the show.

“Part of the news this morning that made me feel really good is seeing how every half hour built,” Ostroff said. “In the 11th hour, I wondered about whether this was the right decision to put the two hours back to back. It’s a lot of drama. But people really liked these characters.”

Also: In its second week in its new time period, NBC’s “America’s Got Talent” (10.6 million viewers, 2.8/8) posted a Tuesday low. Fox placed second with repeats, followed by NBC, CBS led by “Big Brother” (6.5 million 2.4/6) and ABC, which aired the finale of “Wanna Bet” (3.9 million, 1.5/4). Convention ratings will be posted later today.