Whole-canopy net CO 2 exchange (NCE C ) was measured near key stages of fruit development in grapevines (Vitis vinifera L. cv. Cabernet Sauvignon) that were managed under three approaches to regulated deficit irrigation (RDI): (1) standard practice (RDI S ), or weekly replacement of 60-70% of estimated evapotranspiration for well watered grapevines; (2) early additional deficit (RDI E ), or one-half of RDI S applied between fruit set and the onset of ripening (veraison), followed by RDI S ; and (3) RDI S followed by late additional deficit (RDI L ), or one-half of RDI S applied between veraison and harvest. Summed between fruit set and harvest, nearly 40% less irrigation was applied to RDI E vines and~20% less to RDI L vines than to those continuously under RDI S . After~5 weeks of additional deficit, NCE C in RDI E vines was 43-46% less per day than in RDI S vines. After RDI L vines had been under additional water deficit for~3 weeks, NCE C was 33% less per day than in RDI S vines. Instantaneous rates of NCE C responded rapidly to irrigation delivery and elapsed time between irrigation sets. Concurrent single-leaf measurements (NCE L ) reflected the relative differences in NCE C between irrigation treatments, and were linearly associated with NCE C (r 2 = 0.61). Despite halving the water applied under commercial RDI, mid-day stomatal conductance values in RDI E and RDI L of~50-125 mmol m -2 s -1 indicated that the additional deficit imposed only moderate water stress. There was no effect of additional deficit on yield or berry maturity.