  The study of massive star formation needs some basic tools, among which
reliable SFR tracers are crucial ones. We are presently revising the
calibration of SFR tracers at different wavelength ranges using last generation
evolutionary population synthesis codes. The FIR luminosity produced by the
heated dust in star forming regions is commonly used to characterize a
starburst. The X-ray luminosity has been found to show a narrow correlation
with it, and therefore it has been proposed as an additional tracer of
potential interest for high $z$ galaxies, whose FIR emission is redshifted to
the sub-mm range. In this communication we analyze the evolution of the X-ray
to FIR luminosities ratio as predicted by the CMHK population synthesis models,
both for constant stellar formation and instantaneous bursts. The results are
compared to some sample data taken from the literature. The conclusion drawn
from the comparison is that the empirical calibration of the soft X-ray
luminosity seems a valid SFR tracer only for starbursts around a rather short
period of time.
