Dimers and trimers of adenosine monophosphate (AMP) were synthesized from AMP in environments simulating hot vents on the sea floor of the primitive Earth. The simulated environments were made in the flow reactor, in which an aqueous solution of reactants was circulated from the hot to the cold region repeatedly. The oligomerization proceeded most significantly when the hot reaction solution at about 110 degrees C was abruptly ejected into the cold environment maintained at about 0 degree C.