Measurements of hydroxyl (OH) and hydroperoxy (HO₂) radicals were made during the Mexico City Metropolitan Area (MCMA) field campaign as part of the MILAGRO (Megacity Initiative: Local and Global Research Observations) project during March 2006. These measurements provide a unique opportunity to test current models of atmospheric RO_x (OH + HO₂ + RO₂) photochemistry under polluted conditions. A zero-dimensional box model based on the Regional Atmospheric Chemical Mechanism (RACM) was constrained by 10-min averages of 24 J -values and the concentrations of 97 chemical species. Several issues related to the RO_x chemistry under polluted conditions are highlighted in this study: (i) Measured concentrations of both OH and HO₂ were underpredicted during morning hours on a median campaign basis, suggesting a significant source of radicals is missing from current atmospheric models under polluted conditions, consistent with previous urban field campaigns. (ii) The model-predicted HO₂/OH ratios underestimate the measurements for NO mixing ratios higher than 5 ppb, also consistent with previous urban field campaigns. This suggests that under high NO_x conditions, the HO₂ to OH propagation rate may be overestimated by the model or a process converting OH into HO₂ may be missing from the chemical mechanism. On a daily basis (08:40 a.m.–06:40 p.m.), an analysis of the radical budget indicates that HONO photolysis, HCHO photolysis, O₃-alkene reactions and dicarbonyls photolysis are the main radical sources. O₃ photolysis contributes to less than 6% of the total radical production.