Metaphysical rationalism is the view that, necessarily, every fact that stands in need of a metaphysical (grounding) explanation has one. Varieties of metaphysical rationalism include classical theism, Spinozism, spacetime priority monism, and axiarchism. Grounding indeterminism is the view that the same ground, in precisely the same circumstances, might not have grounded what it in fact grounds. I argue that a plausible defense of any form of metaphysical rationalism requires a commitment to grounding indeterminism.