The solar ultraviolet irradiance measurements in the 120-400 nm wavelength range are reviewed and compared showing still important discrepancies between the irradiance values deduced from the most recent observations. The possible variations of the solar ultraviolet irradiances with the 27-day rotation period of the Sun and with the 11-year activity cycle are presented and discussed on the basis of the available irradiation fluxes obtained during the rising phase of solar cycle 21. The spectral features of both kinds of variation are clearly related to the solar atmospheric layer from which the corresponding radiation is emitted.