By Jan Rocha
BBC, Sao Paolo
Brazil's Vice-President Jose Alencar has signed a provisional measure or decree allowing farmers to plant genetically-modified seeds.
Environmental groups are expected to challenge the move
It contains a number of safeguards to limit their use.
But environmental groups are expected to try and challenge the decree in the courts.
The presidential measure lays down strict conditions for the planting and sale of GM crops.
Only farmers who already GM seeds will be allowed to plant them, which effectively means only farmers in Brazil's southernmost state of Rio Grande do Sul.
They have been smuggling them in from neighbouring Argentina, where GM crops are legal.
Farmers will have to sign a term of responsibility which includes financial responsibility for any potential damage to health or the environment.
The planting and sale of GM crops will only be permitted until the next season which begins in October.
By the time this crop is harvested next year, the government hopes that the national congress will have passed a new law defining once and for all Brazil's policy on GM crops.
At the moment, Brazil is the only large agricultural exporter in the world which does not use GM seeds.
But environmental organisations want to keep it that way and are now expected to take the question to the courts, claiming that the presidential decree is unconstitutional.