Ibrahim Sharif, the leader of the secular party Wa'ad, Hassan Mushaima and Abduljalil Singace of the Shia party Haq and the prominent human rights activist Abdulhadi al-Khawaja are all serving long prison sentences.
They were convicted of trying to overthrow the government but international observers believe they are innocent of all charges.
There is strong evidence that they were tortured in detention.
The opposition say that releasing the men would be a sign that the king was serious about reform.
They also say that the more than 1,000 people - almost all Shia - who were sacked from their jobs for supporting calls for reform should be immediately re-instated.
"That's not happening," Mansoor al-Jamri told me. Mr Jamri is the editor of al-Wasat, the only independent newspaper in Bahrain. He called the sacking of Shia a form of apartheid.
"Releasing political prisoners, restoring people's jobs - these are easy steps, if the king is serious about meaningful change," the activist told me.