The effect of water stress, commencing from the late cell division period, on in vivo grain growth was studied in relation to grain sucrose, water status and in vitro starch synthesis. Detached ear experiments were conducted to assess the effect of sink dehydration on grain filling processes under non-limiting source conditions. Water stress caused premature grain desiccation and resulted in a marked decline in grain sucrose and reduced grain weight. Both sucrose uptake and conversion to starch in vitro were increased by mild water stress (solute potential (Ψs)−0·8 MPa). However, a decline in Ψs below this optimum resulted in reduced sucrose uptake and starch synthesis not attributable to a reduced supply of sucrose. Stressed grains which failed to accumulate dry matter in vivo showed significant starch synthesis when cultured in vitro. Grains from in situ and osmotically stressed plants showed a lower capacity for starch synthesis in vitro. The results indicate that grain filling processes under stress conditions are limited by (1) low substrate availability and low Ψs within the sink i.e. an unfavourable seed environment (non-lasting effect) and (2) reduced synthetic capacity of the sink (carry-over effect).