Text messages obtained by newspaper
Mrs Brooks told the Leveson Inquiry earlier this year that Mr Cameron signed some of his missives to her 'LOL' - mistakenly thinking it meant 'Lots of Love' rather than 'Laugh Out Loud'.
Both of the messages disclosed by the MoS were sent in October 2009, shortly after Mrs Brooks left her job as editor of The Sun and became chief executive of News International, which owns the paper.
LOL? More texts from PM revealed
In one message obtained by the Mail on Sunday, the Prime Minister thanked Mrs Brooks for letting him ride one of her horses, joking it was "fast, unpredictable and hard to control but fun".
In another the journalist, who faces trial in connection with the phone hacking scandal, praised Mr Cameron's speech to Tory conference, saying: "I cried twice."
The playful texts are apparently part of a cache of texts and emails handed to Lord Justice Leveson's media standards inquiry.