An individual flower-painting technique that utilises N6- benzylaminopurine (BAP) to increase pod set was used on an indeterminate cultivar of narrow-leafed lupin (Lupinus angustifolius L. cv. Merrit) and on a breeding line with restricted branching (84A/241) to examine the interaction between pod set, water deficit, and seed yield. Petals and sepals of each flower on each inflorescence were painted with a 0.002 M solution of BAP every day from the first day the flower opened to the day it senesced. A water deficit was induced, after flowering on the first-order apical branch, in half of the plants. The other half were maintained at a soil water content close to field capacity. Leaf water potential and leaf conductance declined and remained at about -1.25 MPa and 300 mmol/m2 · s, respectively, in the treatments in which a terminal water deficit was induced. Application of BAP to flowers had no effect on plant-water relations. The water deficit reduced the number of pods that reached maturity (mature pods) when no BAP was applied and increased seed yield in pods that filled seeds. Application of BAP increased the number of pods that reached maturity. However, the additional pods that reached maturity produced unfilled seeds. Seed yield and harvest index were reduced in the BAP treatments, mainly as a result of a reduction in seed number. An increase in seed abortion during seed filling probably caused the reduction in seed number. We conclude that the reduction in seed number and pod filling resulted from a shortage of assimilates to fill all the mature pods produced.