The severe shortage of deceased-donor kidneys has turned the allocation into a rationing problem. Previous research and policy guidance on the design of allocation system often makes restrictive assumptions about patient behavior. This study provides quasi-experimental evidence on how patients respond to allocation rules. We exploit a national kidney allocation policy that assigns priority based on an exogenous cutoff. Allocation priority produces a positive supply shock: prioritized patients receive more frequent kidney donors with generally higher quality. Using a sharp regression-discontinuity design, we estimate that this supply shock has reduced the likelihood of seeking a living donor by a factor of 52.8%. In addition, we find that transplant candidates become more selective; they are less likely to accept an organ donor of a given quality. Such behavioral responses can lead to an increasing number of organs being discarded, further exacerbating the organ shortage. Our results emphasize the need to incorporate patient behavior when prospectively evaluating the performance of alternative allocation system. These findings also have implications on kidney allocation policy making.