Abscisic acid (ABA; free form) is a naturally occurring physiological growth hormone of higher plants. A detailed study involving the time course growth of developing seed tissues associated with endogenous levels of free ABA were investigated using a novel enzyme-linked immunosorbent assay. Seed filling in castor (Ricinuc communis L.) endosperm, embryo, and pod is marked with a rapid increase in fresh weight during the mid-developmental stages [21-42 days after pollination (DAP)], followed by a steady decline at the maturation stages (42-63 DAP) accompanied with a rapid lipid synthesis (in endosperm and embryo) during the same period, except for in pod. Endogenous ABA levels in endosperm (0.001-0.32 mu g/g) and embryo (0.003-0.13 mu g/g) followed a concurrent pattern with seed reserve filling, showing a rapid increase during the mid-developmental stages 21-42 DAP, whereas ABA levels in seed pod (0.2-22.9 mu g/g) showed a different accumulation pattern with rapid increase and decline during the early-mid developmental stages, preceded by the maximal increase during the maturation stage (63 DAP). Together, our results provide evidence for the association of endogenous ABA in seed filling as well as in reserve deposition and provides clue for the effective usage of exogenous ABA concentrations in developing seeds with a focus, on improving seed reserve complex in castor.