Protection of participants - Participants were exposed to extremely stressful situations that may have the potential to cause psychological harm. Many of the participants were visibly distressed.
Signs of tension included trembling, sweating, stuttering, laughing nervously, biting lips and digging fingernails into palms of hands. Three participants had uncontrollable seizures, and many pleaded to be allowed to stop the experiment.
Full blown seizures were observed for 3 participants; one so violent that the experiment was stopped.
In his defence, Milgram argued that these effects were only short term. Once the participants were debriefed (and could see the confederate was OK) their stress levels decreased. Milgram also interviewed the participants one year after the event and concluded that most were happy that they had taken part.