A 65-year-old woman and her son from the Chichester area have been arrested after police suspected they were planning to take her husband to the Dignitas clinic in Switzerland to commit suicide, sources said.
The pensioner and the 25-year-old were held on suspicion of encouraging or assisting a suicide.
Details about the condition of the 71-year-old man have not been disclosed and none of the family have been named by Sussex Police.
But the force confirmed that officers were having the mental capacity of the “vulnerable” man assessed to determine how able he is to make decisions for himself.
Both the woman and her son have been freed on police bail until October 8 following their arrest on August 8.
A Sussex Police spokesman said: “Police have been made aware of suggestions that a man and a woman from West Sussex could be planning to take a vulnerable pensioner to the Dignitas clinic in Switzerland so that he can end his life.
“Officers are currently having the mental capacity of the vulnerable 71-year-old assessed to determine how able he is to make decisions for himself.
“A 25-year-old man and a 65-year-old woman from the Chichester area were arrested on Thursday, August 8, on suspicion of encouraging or assisting a suicide.
“Both have been released on bail without charge until 8 October while officers carry out further inquiries.
“It is an offence to encourage or assist suicide under the Coroners and Justice Act 2009 and officers are investigating whether any crime has been committed or is likely to be committed if they do not take action.”
Guidelines were issued in February 2010 by the Director of Public Prosecutions Keir Starmer, QC which clarified the position on assisted suicide.
It was indicated that anyone acting with compassion to help end the life of someone who has decided they cannot go on would be unlikely to face criminal charges.
Assisted suicide remains a criminal offence in England and Wales, punishable by up to 14 years in prison, but individual decisions on prosecution are now made on the circumstances in each case.
The guideline document was published after a Law Lords ruling in favour of MS sufferer Debbie Purdy who wanted to know whether her husband would be prosecuted for helping her end her life.
Dignitas is a Switzerland-based assisted dying group which over the past 14 years has helped more than 1,100 people to die. More than 150 Britons have chosen to die at its clinic in Zurich.
A total of 200 people a year end their life there because assisted suicide is legal in Switzerland, as long as the helper does not personally benefit from the death.
The Swiss government has tried to reduce so-called suicide tourism. However, in May 2011, about 85% of voters in Zurich rejected a call to end legalised assisted suicide, while 78% rejected a call to ban foreigners from going to the country to end their life.