  The age-metallicity relation of the solar neighbourhood is studied using a
sample of 552 late-type dwarfs. This sample was built from the intersection of
photometric catalogues with chromospheric activity surveys of the Mount Wilson
group. For these stars, metallicities were estimated from uvby data, and ages
were calculated from their chromospheric emission levels using a new
metallicity-dependent chromospheric activity-age relation developed by
Rocha-Pinto & Maciel (1998). A careful estimate of the errors in the
chromospheric age is made. The errors in the chromospheric indices are shown to
include partially the effects of the stellar magnetic cycles, although a
detailed treatment of this error is still beyond our knowledge. It is shown
that the results are not affected by the presence of unresolved binaries in the
sample. We derive an age-metallicity relation which confirms the mean trend
found by previous workers. The mean metallicity shows a slow, steady increase
with time, amounting at least 0.56 dex in 15 Gyr. The initial metallicity of
the disk is around -0.70 dex, in agreement with the G dwarf metallicity
distribution. According to our data, the intrinsic cosmic dispersion in metal
abundances is around 0.13 dex, a factor of two smaller than that found by
Edvardsson et al. (1993). We show that chromospheric ages are compatible with
isochrone ages, within the expected errors, so that the difference in the
scatter cannot be caused by the accuracy of our ages and metallicities. This
reinforces some suggestions that the Edvarsson et al.'s sample is not suitable
to the determination of the age-metallicity relation.
