A three-component model has been developed to examine the variation with solar activity of the far ultraviolet irradiance between 145 and 200 nm. This model is based on spatially resolved observations of the Call K chromosphere and includes the contributions to the full disk flux from both plage and active network emission. The 27-day modulation of the ultraviolet flux is explained by the evolution and rotation of the plage regions on the solar disc. Over the longer time scale of the eleven-year cycle it is essential that changes in the active network arising from the decay of plage regions also be solar flux is it possible to simultaneously reproduce the 27-day variability observed by the solar backscatter ultraviolet experiment on the Nimbus 7 satellite and the changes from the minimum to the maximum of the solar activity cycle observed by the rocket experiments of the Laboratory for Atmospheric and Space Physics and by the extreme ultraviolet spectrometer on the Atmospheric Explorer E satellite. It is shown that the AE-E experiment measured a smaller solar cycle variability for the ultraviolet irradiances than is predicted by the model calculations because of the spatially restricted field of view of this instrument.