  We use a series of ray-tracing experiments to determine the magnification
distribution of high-redshift sources by gravitational lensing. We determine
empirically the relation between magnification and redshift, for various
cosmological models. We then use this relation to estimate the effect of
lensing on the determination of the cosmological parameters from observations
of high-z supernovae. We found that, for supernovae at redshifts z<1.8, the
effect of lensing is negligible compared to the intrinsic uncertainty in the
measurements. Using mock data in the range 1.8<z<8, we show that the effect of
lensing can become significant. Hence, if a population of very-high-z
supernovae was ever discovered, it would be crucial to fully understand the
effect of lensing, before these SNe could be used to constrain cosmological
models.
