RESTRICTIONS ON PITCHERS WARMING UP
The home club pitcher may not warm up on the mound prior to the start of the game. To do otherwise could give the home club pitcher some advantage, and because pregame practice conditions should be equal for both starting pitchers, both pitchers should warm up where pitchers normally warm up.
A game pitcher may return to the bullpen between innings, provided doing so does not delay the game in any manner.
The Standards and On-Field Operations Regulation 2-7 (“Pace of Game Procedures”) provides the following regarding warm-up pitches:
“The pitcher may take as many warm-up pitches as he desires prior to the inning, but regardless of how many warm-up pitches he has thrown, he must deliver his final warm-up pitch at least 20 seconds prior to the end of an inning break or pitching change. The umpire shall signal for the last warm-up pitch at 25 seconds, unless a special circumstance applies.”