The singer was adamant that she wouldn't miss a phone call with Mick Jagger regarding her appearance at the band's 50th anniversary show, so spoke to him while having a bath.
"GaGa didn't want to miss Mick's call so she took it in the bath," a source told The Sun. "They spoke at length about what's going to happen.
"She also asked Mick if she could wear any of his old stage gear for their performance."
GaGa will join Bruce Springsteen and The Black Keys for The Rolling Stones' final 'One More Shot' show at Newark's Prudential Centre in New Jersey on December 15.
The iconic band have previously been joined by Florence Welch, Mary J Blige and Eric Clapton.
Watch The Rolling Stones perform with Florence Welch at London's O2 Arena last month below: