As measured by the Solar Mesosphere Explorer satellite, the density of nitric oxide at low latitudes (30¿S to 30¿N) and at 110 km (E-region) decreased from a mean value of 3¿107 molecules/cm3 in January 1982 to a mean value of 4¿106 molecules/cm3 in April 1985. In addition, the nitric oxide density varied ¿50% with a 27-day period during times of high solar activity. The variation of nitric oxide correlates with variations in the solar Lyman-alpha irradiance which is also measured by the Solar Mesosphere Explorer satellite. The Lyman alpha irradiance is interpreted as an index of the variations in the solar EUV and soft X-ray flux. The hypothesis is proposed that the solar X-ray flux between 20 and 100 ¿ has a larger variation than the solar EUV flux between 100 and 1050 ¿ and that the solar X-rays produce photoelectrons which are the source of the nitric oxide. ¿ American Geophysical Union 1988