  We have studied the sensitivity of s-process nucleosynthesis in massive stars
to +/- 2 sigma variations in the rates of the triple alpha and 12C(a,g)16O
reactions. We simulated the evolution of massive stars from H-burning through
Fe-core collapse, followed by a supernova explosion. We found that: the
production factors of s-process nuclides between 58Fe and 96Zr change strongly
with changes in the He burning reaction rates; using the Lodders (2003) solar
abundances rather than those of Anders and Grevesse (1989) reduces s-process
nucleosynthesis; later burning phases beyond core He burning and the shell C
burning have a significant effect on post-explosive production factors. We also
discuss the implications of the uncertainties in the helium burning rates for
evidence of a new primary neutron capture process (LEPP) in massive stars.
