  We have worked out evolutionary synthesis models of the broad-band spectral
energy distribution of elliptical galaxies over the whole frequency range from
UV to far--IR. Internal extinction and far--IR re--emission by interstellar
dust have been taken into account in a self--consistent way. Diffuse dust
emission has been modelled in terms of two components: warm dust, located in
regions of high radiation intensity, and cold dust, heated by the general
radiation field. Emission from circumstellar dust clouds at different galactic
ages was also taken into account. The models reproduce well the present average
broad--band spectrum of nearby ellipticals over about four decades in
frequency. Under the assumption of a dust--to--gas ratio proportional to the
metallicity, the fraction of bolometric luminosity coming out at far-IR
wavelengths strongly evolves during the galaxy lifetime, ranging from very low
local values ($\lsim 0.5\%$) to $\sim 30\%$ or more in the first billion years.
Some models even imply that early phases are optically thick; these models
provide an excellent fit of the observed spectral energy distribution of the
high redshift galaxy IRAS F10214$+$4724. Far--IR observations may thus play a
key role in investigating the early evolution of elliptical galaxies. A strong
far-IR evolution of early-type galaxies might be a crucial ingredient to
explain the deep $60\mic$ IRAS counts.
