Thermospheric concentrations of neutral atomic hydrogen near and below the F peak are directly related to H+, O+ and atomic oxygen concentrations through the charge exchange equilibrium that is established between hydrogen and oxygen at these altitudes. This chemical relationship, together with in situ measurements of ionospheric and neutral atmospheric concentrations by instrumentation on board the Atmosphere Explorer C satellite, is utilized to investigate properties of neutral hydrogen at altitudes below 200 km where vertical diffusion strongly affects the hydrogen distribution. Data are discussed for a set of satellite orbits during quiet geomagnetic and solar conditions in February 1974; the resultant altitude variation of the derived hydrogen concentrations applies specifically to early afternoon at low 105 atoms/cm3 is observed for these conditions at 300 km. At lower altitudes the concentration profiles are interpreted in terms of vertical hydrogen flow. The resultant daytime flux in the thermosphere is estimated to be (3.2¿1.0) ¿108 atoms/cm2 s. The present observations thus support theoretical estimates and model calculations of large hydrogen flow upward from the region below 100 km. They also support the concept of daytime thermospheric loss process of greater magnitude than the traditional evaporative escape mechanism.