This paper estimates the patterns and sources of White-Black and White-Hispanic enrollment gaps in Advancement Placement (AP) and dual enrollment (DE) programs across several thousand school districts and metropolitan areas in the U.S. By merging several data sources, we show that both AP and DE enrollment gaps vary substantially across districts. We find that the vast majority of districts have racial/ethnic gaps in AP and DE participation, and about a quarter of districts have racial/ethnic gaps equal to or larger than 10 and 7 percentage points for AP and DE, respectively. Available district-level characteristics and state-level policies explain much more of the geographic variation in AP enrollment gaps as compared to DE enrollment gaps, and local factors (either district-level or metro-level characteristics) dominate state-level factors in shaping these racial/ethnic participation gaps. Among all the available district-level characteristics, racial/ethnic composition and racial/ethnic income disparity are the strongest correlates of participation gaps, where districts with larger proportions of Black and Hispanic students and greater racial/ethnic income disparity are associated with larger racial/ethnic gaps in both AP and DE enrollment.