Productive and metabolic responses are supported by body reserves changes in beef cows when facing feed restriction (FR). Because dynamics of natural abundance of 13C (δ13C) and 15N (δ15N) in milk may reflect body changes in dairy cows, the objectives were i) to test if plasma and milk isotope signatures change in response to FR in beef cows and ii) to study their associations with phenotypic feed efficiency. Twenty-two Charolais cows were studied from calving until wk 21 of lactation. Residual feed intake (RFI) was calculated on wks 1 to 8 of lactation when cows were allowed ad libitum intake of the same diet. On wk 9, cows underwent a 4-day FR to meet 50% of net energy requirements, followed by ad libitum intake. Individual production (MY) and metabolic (NEFA) responses were monitored from d-10 to d19 relative to FR. The δ13C and δ15N were analyzed in plasma [before (d-10, d-2, d1); during (d2-5) and after (d6-d8, d11 and d19) FR] and in milk [before (d-10, d-2, d1); during (d4, d5) and after (d11, d19) FR]. The effect of FR and RFI on plasma and milk δ13C and δ15N were tested using mixed model and Spearman correlations. RFI ranged from -0.01 to -0.69 for negative RFI and from 0.04 to 1.15 kg DMI/d for positive RFI and have no effect on plasma and milk isotopic signatures. Plasma NEFA and δ15N increased, and δ13C decreased during FR (P<0.0001). Accordingly, milk δ15N increased and δ13C decreased during FR (P<0.0001). Plasma NEFA was correlated across the experiment with plasma δ15N (r=0.41, P<0.0001) and δ13C (r=0.40, P<0.0001), whereas no correlation was shown with milk isotopes signatures. Plasma δ15N was correlated with milk δ15N (r=0.66, P<0.001), but no relation was observed between plasma and milk δ13C. These results suggest that only plasma isotope signatures respond to body reserves changes in Charolais cows but correlations with plasma NEFA were moderate. To date, no clear relations with RFI were found. Further work is ongoing to model kinetics of δ13C and δ15N to explore links with feed efficiency.