Differences in the chemical composition of meat from lambs fed contrasted diets, for example pasture vs a concentratebased diet indoors, are well known. They are the basis of authentication methods to discriminate between pasture-fed and concentrate-fed lamb meat. However, the question of the timing of appearance of markers of pasture-feeding in lamb meat relative to the start of pasture-feeding is not known. The objective of the present study was therefore to study the kinetics of change in lamb meat composition and to determine whether it was possible to discriminate between different grazing durations in order to authenticate lamb meat production conditions. Four groups of 55 Romane lambs were used: L0, concentrate-fed lambs, no grazing; L21, L42 and L63, with lambs grazing alfalfa for 21, 42 and 63 days before slaughter, respectively. At slaughter, measurements of the commercial and sensory qualities of the carcass and meat were performed, as well as spectral analysis of two fat tissues (perirenal and dorsal fat, PF and DF) and of the longissimus dorsi (LD) muscle using visible and infrared spectroscopy. Regarding authentication purposes: (1) PF skatole concentration (skatole is a compound both responsible for off-flavour and proposed as a marker of grass feeding in some publications) did not allow for a sufficiently reliable classification of pasture-fed lambs vs concentrate-fed lambs; (2) the combination of PF skatole concentration and an indicator of PF carotenoid concentration, using a decision tree, enabled to correctly discriminate L0 lambs from the pasture-fed (L21, L42, L63) lambs, but not to authenticate the duration of pasture-finishing; (3) discriminant analysis on the visible spectrum of PF or DF also allowed to discriminate L0 lambs from the pasture-fed (L21, L42, L63) lambs, but not to authenticate the duration of pasture-finishing. The LD spectral data did not allow to discriminate L0 lambs from L21 lambs and L42 lambs from L63 lambs, but they enabled to reliably discriminate L0 and L21 lambs from L42 and L63 lambs. The perspectives are to expand the spectral range explored using infrared spectroscopy.