During Leg 37, Deep Sea Drilling Project, several holes were drilled on the northwest flank of the Mid-Atlantic Ridge. Calcareous nannofossils can be used to define age limits for the upper portion of basement rocks at sites 332 and 334. Nannofossil oozes recovered from immediately above basement basalts in holes 332A and 332B are of early Late or late Early Pliocene age (Discoaster tamalis or D. asymmetricus Subzone). Nannofossil chalks recovered from as deep as 180 m below the top of basalt in these holes are of late Early Pliocene age (D. asymmetricus Subzone). Nannofossils recovered from sediments immediately overlying basement basalts in hole 334 are of early Late Miocene age (D. bellus Subzone). Nannofossil chalks recovered from 50 m below the top of basalt in this hole are of late Middle Miocene age (Catinaster calyculus Subzone). Nannofossils recovered from the calcareous matrix of a gabbro and periodotite breccia occurring 68 m below the top of the basalt are also of late Middle Miocene age (C. coalitus Zone).The results indicate that the upper 180 m of basalt at site 332 were formed during an interval of time not exceeding 1 million years. At site 334 the upper 50 m of basement basalts were formed during an interval of time not exceeding 4.5 million years.