Chavez will carry out "adjustments, not changes" in his cabinet, he told Unión Radio news network, declining to give further details.
The ministers will stay in their current posts until replacements are assigned and approved.
The president has shuffled his ministers in the past but never before has asked all to resign.
Ramirez has been minister since 2002 and was appointed president of state oil company PDVSA in November 2004.
PDVSA officials confirmed news of the resignation to BNamericas, adding Ramirez did not resign as PDVSA president.
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