Total suspended particulate (TSP) samples have been collected at six stations in the C and B lines of the Buenos Aires underground system and, almost simultaneously, at six ground level sites outside and nearby the corresponding underground stations, in the Oct 2005/Oct 2006 period. All these samples were analyzed for mass and elemental Fe, Cu, and Zn concentrations by using the Particle Induced X-ray Emission (PIXE) technique. Mostly, TSP concentrations were found to be between 152 µg m^-3 (25% percentile) and 270 µg m^-3 (75% percentile) in the platform of the stations, while those in outside ambient air oscillated from 55 µg m^-3 (25% percentile) to 137 µg m^-3 (75% percentile). Moreover, experimental results indicate that TSP levels are comparable to those measured for other underground systems worldwide. Statistical results demonstrate that subway TSP levels are about 3 times larger on average than those for urban ambient air. The TSP levels inside stations and outdoors are poorly correlated, indicating that TSP levels in the metro system are mainly influenced by internal sources. Regarding metal concentrations, the most enriched element in TSP samples was Fe, the levels of which ranged from 36 (25% percentile) to 86 µg m^-3 (75% percentile) in Line C stations, while in Line B ones they varied between 8 µg m^-3 (25% percentile) and 46 µg m^-3 (75% percentile). As a comparison, Fe concentrations in ambient air oscillated between 0.7 µg m^-3 (25% percentile) and 1.2 µg m^-3 (75% percentile). Other enriched elements include Cu and Zn. With regard to their sources, Fe and Cu have been related to processes taking place inside the subway system, while Zn has been associated with outdoor vehicular traffic. Additionally, concerns about possible health implications based on comparisons to various indoor air quality limits and available toxicological information are discussed.