The 24 male students were randomly assigned to be either a prisoner or guard. Guards were allowed to make up rules and prisoners had to wear a uniform with a number on which they could only refer to each other as. Prisoners also had to wear an ankle chain and a stocking cap to eliminate individuality.
On the second morning, the prisoners rebelled and barricaded themselves in their cells. The guards forced themselves in and strip-searched the prisoners. Ring-leaders were put in solitary confinement. Those least involved given washing privileges.
The guards became even more brutal. They refused to let prisoners leave their cells and one prisoner even showed signs of anxiety despite being regarded as emotionally stable prior to the experiment. They weren't allowed to leave until Day 4.
One guard was thought to have been particularly brutal and he was nicknamed 'John Wayne'. This participant clearly took on the role of being a guard intensely.
The guards made the prisoners do horrible acts like cleaning toilets with their bare hands and also sexually humiliating prisoners.
The guards had almost total power and the prisoners obeyed without question. "Tyranny was inevitable" said Zimbardo. The experiment had to be stopped after only 6 days.