The cast of "Friends" (search) began full-scale rehearsals yesterday for its final episode - set to be taped Friday night.

Insiders say the show's stars are striving to keep their final week "typical" of the series' usual routine.

But for millions of viewers, the show, slated to air next May, will be anything but typical when Monica, Chandler, Joey, Ross, Rachel and Phoebe say goodbye in a send-off that may draw up to 40 million people - the biggest since "Seinfeld" (search) bid farewell in 1998.

"They had Monday off [for the Martin Luther King holiday]," said a source. "Tuesday and Wednesday, they'll have rehearsals, Thursday and Friday they'll rehearse in front of the cameras and then late Friday they'll tape the show.

"It's a pretty typical work week," the source told The Post.

Parts of the episode will be filmed in front of a studio audience and other parts - most likely the final few scenes - will be filmed in front of an empty gallery, in order to preserve the secrecy.

Producers are even rumored to be contemplating filming several endings, and then choosing one shortly before the finale airs on May 6.

Security on the set is expected to be intense, and producers are said to be sensitive about details of the episode leaking out.

In the weeks leading up to the finale of "Seinfeld" several series finale scripts which were supposed to be leaked from inside the studio were posted on the Internet.

None turned out to be real.

An episode of "Friends" usually takes about six hours to film due to wardrobe changes, make-up changes and occasional script rewrites.

The one-hour finale episode could take much longer.

Even with five months to go before it airs, NBC has already begun promoting the special night of programming. The evening starts with an hour-long clip show featuring classic scenes from the show's 10-year history. It will air at 9 p.m., followed by the hour-long finale at 10 p.m.

Later, "The Tonight Show with Jay Leno" (search) will be devoted to the sitcom and take place with the cast of "Friends" on the set of Central Perk, the coffee shop where the characters usually hang out.

Commercial airtime during the finale is reportedly going for Super Bowl (search) sized rates, around $2 million, while its stars have reportedly taken home roughly $42 million each over the course of the last two seasons.