Metal foil collectors were installed on the external structure of Skylab to entrap precipitating magnetospheric particles. The foils were retrieved and the entrapped helium, neon, and argon were isotopically analyzed in a high-resolution mass spectrometer. Solar and terrestrial helium and neon and terrestrial argon were detected. As expected, the isotopic composition of neon and argon in the high atmosphere was found to be strongly fractionated. Special techniques were used to estimate the initial particle energy of 3He. The measured 1He flux is consistent with the assumption that precipitating solar 3He is the major source of terrestrial 3He.