  The stellar initial mass function and the stellar lifetimes are basic
ingredients of chemical evolution models, for which different recipes can be
found in the literature. In this paper, we quantify the effects on chemical
evolution studies of the uncertainties in these two parameters. We concentrate
on chemical evolution models for the Milky Way, because of the large number of
good observational constraints. Such chemical evolution models have already
ruled out significant temporal variations for the stellar initial mass function
in our own Galaxy, with the exception perhaps of the very early phases of its
evolution. Therefore, here we assume a Galactic initial mass function constant
in time. Through an accurate comparison of model predictions for the Milky Way
with carefully selected data sets, it is shown that specific prescriptions for
the initial mass function in particular mass ranges should be rejected. As far
as the stellar lifetimes are concerned, the major differences among existing
prescriptions are found in the range of very low-mass stars. Because of this,
the model predictions widely differ for those elements which are produced
mostly by very long-lived objects, as for instance 3He and 7Li. However, it is
concluded that model predictions of several important observed quantities,
constraining the plausible Galactic formation scenarios, are fairly robust with
respect to changes in both the stellar mass spectrum and the stellar lifetimes.
For instance, the metallicity distribution of low-mass stars is nearly
unaffected by these changes, since its shape is dictated mostly by the time
scale for thin-disk formation.
