Correspondents say the letter read on state TV is the first suggestion that the 81-year-old president may give up his position in Cuba's leadership.
Last year he handed over power to his brother Raul on a temporary basis.
Since then, he has not made a public appearance - although the government has released pictures of Mr Castro.
In the letter released on Monday, he said his duty was not to cling to office but "to pass on experiences and ideas whose modest value arises from the exceptional era in which I lived".
The message was delivered during Cuba's main nightly current affairs programme, Mesa Redonda.
Fidel Castro has ruled Cuba since leading the 1959 revolution.
Earlier this month he was nominated as a candidate for a seat in Cuba's National Assembly - a move seen as an indication that he might still hope for a return to power.
Mr Castro must be re-elected to the assembly if he is to remain president of the Council of State.
Nationwide elections will be held on 20 January.
The newly elected parliament will then choose the Council of State, which President Fidel Castro has headed since the early 1960s.
Güncelleme Tarihi: 18 Aralık 2007, 17:51