Incorporating the particular features of the U.S. tax code, notably corpora
te interest deductions and R&D expensing, into Romer's R&D-based endogenous
growth model, we analyze how taxation of different-source capital income a
ffects long-run growth by distorting saving, input demands, and stock marke
t P/E ratios. Surprisingly, we find that a pro-growth tax system requires a
lower income tax rate for noninnovative firms, particularly when R&D recei
ves taw credits. Simulations for the U.S. economy provide the effects of ta
xation on growth, interest rates, P/E ratios, human capital allocations, an
d welfare.