Mass loading of the Neptunian magnetosphere occurs primarily by thermal escape of H, H2, and N from Triton's upper atmosphere. The global escape rate of hydrogen is ~7¿1025 s-1, determined by the global average methane photolysis rate, whereas the escape rate of nitrogen for our preferred model is ~3.4¿1025 s-1, and is controlled by the global and orbital average energy deposition rate due to precipitating magnetospheric electrons. The escape rate of H+ and N+ is <4% of the neutral escape rate and implies that mass loading of the Neptunian magnetosphere is not localized to Triton's corona. The ratio of hydrogen to nitrogen escape rates for our preferred model is ~2:1, comparable to the [H+>/[N+> abundance ratio inferred for Neptune's magnetosphere. ¿American Geophysical Union 1991