  Radio galaxies with a projected linear size > 1 Mpc are classified as giant
radio sources. According to the current interpretation these are old sources
which have evolved in a low-density ambient medium. Since radiative losses are
negligible at low frequency, extending spectral ageing studies in this
frequency range will allow to determine the zero-age electron spectrum injected
and then to improve the estimate of the synchrotron age of the source. We
present Very Large Array images at 74 MHz and 327 MHz of two giant radio
sources: 3C35 and 3C223. We performed a spectral study using 74, 327, 608 and
1400 GHz images. The spectral shape is estimated in different positions along
the source. The radio spectrum follows a power-law in the hot-spots, while in
the inner region of the lobe the shape of the spectrum shows a curvature at
high frequencies. This steepening is in agreement with synchrotron aging of the
emitting relativistic electrons. In order to estimate the synchrotron age of
the sources, the spectra have been fitted with a synchrotron model of emission.
Using the models, we find that 3C35 is an old source of about 143 Myr, while
3C223 is a younger source of 72 Myr.
