Italy defender Leonardo Bonucci has rejoined Serie A champions Juventus from AC Milan, while striker Gonzalo Higuain has moved the opposite way.
AC Milan will pay 18m euros (£16m) for the loan of Higuain and have the option to make the deal permanent in a year by paying a fee of 36m euros (£32.04m).
Meanwhile, Italy defender Mattia Caldara has left Juve to join AC Milan.
Bonucci, 31, and Caldara, 24, is a swap deal with both transfers worth 35m euros (£31.2m).
Argentina striker Higuain, 30, scored 16 Serie A goals last season.
Higuain, who was linked with a move to Premier League side Chelsea, joined Juve from Napoli for a then domestic record fee of £75.3m in 2016 and scored 40 league goals in two seasons.
AC Milan said in a statement: "Gonzalo and Mattia are two important pieces of the new era of the club."
Gonzalo Higuain was mobbed when he arrived in Milan on Thursda
Bonucci, 31, spent just one season at the San Siro having joined from Juve for £35.1m last summer.
A Juventus statement said: "After spending seven wonderful years in black and white, Leonardo Bonucci has returned to the club following a season at AC Milan.
"Both Juventus and Bonucci are ready to start winning things again together."