Messing, 49, shared cute photos with Mullally, 59, during rehearsals for Season 2 of the NBC revival.
The pictures show Messing and Mullally smiling and making faces for the camera. The photos were taken at Universal Studios Hollywood, where Will & Grace films on Soundstage 22.
"WE'RE BACKKKKKKK!!!! First day of #Season2 for #WillandGrace! It's like the first day of school. @meganomullally @nbcwillandgrace," Messing captioned the post.
Messing and Mullally play Grace Adler and Karen Walker on Will & Grace, which co-stars Eric McCormack and Sean Hayes. Hayes, who portrays Jack McFarland, posted a picture of a table read with the cast and crew.
"Our first table reading of the first script for Season 2 of the 'reboot' of @nbcwillandgrace.... a.k.a, Season 10," the actor wrote. "The first The first episode back is soooooooooooo funny! It's great to be back in school with @eric_mccormack @therealdebramessing @meganomullally #WillAndGrace."
McCormack, who plays Will Truman, playfully responded to Hayes on Twitter.
"Seconded! Kindergarten is in session! @MeganMullally @DebraMessing @MaxMutchnick," he wrote, tagging Will & Grace co-creator Max Mutchnick.
Will & Grace initially had a nine-season run from 1998 to 2006. The series was revived and completed the first season of new episodes in April.