The success of the "primary care case management (PCCM)" form of managed care implemented in many state Medicaid programs over the past several years depends in part on the expanded availability of primary care physician sites to substitute for hospital-based outpatient care and to provide a medical home for enrollees. However, the PCCM requirement for physicians to accept assignment of a caseload of patients and to provide all of their primary care likely conflicts with the approach of limited Medicaid participation favored by many Medicaid physician participants. This study examines the early impact of PCCM implementation, in the absence of physician reimbursement level increases, on the patterns of Medicaid participation by physicians in communities in Georgia and Alabama. We find that the implementation of PCCM under these conditions often was associated with reductions in the proportion of physicians participating in Medicaid, reductions in the number of very small Medicaid practices, and declines in Medicaid visit volumes across all participating physicians. We also find evidence of an overall reduction in the number of primary care visits per Medicaid enrollee, but an increase in the proportion of these visits that were for preventive care services associated with initial PCCM implementation.