In this paper, we investigate in a controlled laboratory experiment physician behavior in the case of payment heterogeneity. In the experiment, each physician provides medical care to patients whose treatments are paid for either under fee-for-service (FFS) or capitation (CAP). We observe that physicians customize care in response to the payment system. A FFS patient receives considerably more medical care than the corresponding CAP patient with the same illness and treatment preference. Physicians over-serve FFS patients and under-serve CAP patients. After a CAP payment reduction in the experiment we observe neither a quantity reduction under CAP nor a spillover into the treatment of FFS patients.