  We present the first results of a project, LSD, aimed at obtaining
spatially-resolved, near-infrared spectroscopy of a complete sample of
Lyman-Break Galaxies at z~3. Deep observations with adaptive optics resulted in
the detection of the main optical lines, such as [OII], Hbeta and [OIII], which
are used to study sizes, SFRs, morphologies, gas-phase metallicities, gas
fractions and effective yields. Optical, near-IR and Spitzer/IRAC photometry is
used to measure stellar mass. We obtain that morphologies are usually complex,
with the presence of several peaks of emissions and companions that are not
detected in broad-band images. Typical metallicities are 10-50% solar, with a
strong evolution of the mass-metallicity relation from lower redshifts. Stellar
masses, gas fraction, and evolutionary stages vary significantly among the
galaxies, with less massive galaxies showing larger fractions of gas. In
contrast with observations in the local universe, effective yields decrease
with stellar mass and reach solar values at the low-mass end of the sample.
This effect can be reproduced by gas infall with rates of the order of the
SFRs. Outflows are present but are not needed to explain the mass-metallicity
relation. We conclude that a large fraction of these galaxies are actively
creating stars after major episodes of gas infall or merging.
