Transplantation is notable for the degree to which resources are allocated via administrative rather than market mechanisms. However, non-monetary incentives still permeate the system. Using instrumental variable regression, we estimate the substitution patterns between cadaveric and living kidney donations in the United States from 1988 to 2008. On average, a decrease of two to five cadaveric donations causes living kidney donations to increase by one. Disaggregating living donors into blood-related and non-blood-related donors, the strongest effect is found among non-blood-related donors known to the organ recipient. A 1% increase of cadaveric donations decreases living donations from this group by 1.54%. (JEL D62, D64, I1)