Many Medicaid policy changes occurred in recent years including those resulting from the Omnibus Budget Reconciliation Act of 1981 and the Tax Equity and Fiscal Responsibility Act of 1982. At the same time, the supply of providers increased and the health care market became more competitive. This paper presents evidence about how these developments are affecting pediatricians' participation in state Medicaid programs. Surveys conducted in 1978 (N = 814) and 1983 (N = 791) indicate that the proportion participating declined only slightly from 85.1% to 82.0%. The average Medicaid case load of participants remained at 15%, although extent of participation of individual pediatricians fluctuated. Previous research demonstrates that physicians' Medicaid participation is affected by reimbursement level, administrative complexity, and generosity of eligibility and benefits. Our data confirm these influences. However, the longitudinal design of the analyses reported here also captures shifts in the relative influence of these factors. The influence of policy factors has diminished over time, while the influence of changes in physician supply has increased. Increased physician supply, however, is associated with decreased Medicaid participation. Thus, diminished access to pediatric care for low-income children may result from recent changes in Medicaid and in the broader health care environment.