Multielemental stable isotope ratio (SIR) analysis was used in lamb plasma, erythrocytes and muscle to detect the switch from a pasture- to a concentrate-based diet, with the aim of verifying the possibility to trace the change of feeding in animal tissues. During 89 days of experimental feeding, lambs were subjected to four dietary treatments: pasture (P), pasture followed by concentrate in the stall for either 14 days (P-S14) or 37 days (P-S37) or concentrate in the stall (S). Pasture and concentrate diets comprised C3 plants only and had different values of 13C/12C, 18O/16O, 2H/1H and 34S/32S ratios. Muscle 13C/12C and 34S/32S and plasma 13C/12C and 18O/16O ratios in P, P-S14 and P-S37 lambs were significantly different. A multivariate analytical approach revealed that 13C/12C and 18O/16O ratios in plasma were the most powerful variables for the discrimination among the dietary treatments.