Upgrade mysql in master to slave environment (no replies)

Hi,

I want to upgrade y mysql version in my Master->Slave environment.
Mysql 5.1.x to last 5.5 version.

I know that i need to upgrade first the slave and next the mast but i am a question about mysql_upgrade.

In where server should I execute mysql_upgrade script? Only the slave, only the amster or both?

Thanks!