The growth and composition of siliquas and seeds of oilseed rape was followed over 12 weeks from shortly after anthesis to maturity. Each plant produced 220 siliquas, this number being constant throughout development. Seed numbers per siliqua fell from 19 to 9 by week 5 and declined to 7 at maturity. Hull(1) and seed growth followed a sigmoid pattern, but were not in phase. Seed development could be divided into 3 phases: In Phase 1, seed weight was low and starch and ethanol soluble compounds accounted for 80% DM. Phase 2, seed growth increased and storage oil and proteins were deposited accounting for 40% and 20% DM respectively at the end of this stage. Starch, glucose and fructose were utilized in this process. Phase 3 was largely concerned with the deposition of oil and protein in fixed proportions. Seed weight more than doubled while DM composition remained constant. Sugars were transferred from the hull to the seed to support this growth.The proportion of hull lipids remained constant throughout development until shortly before maturity when MGDG and DGDG fell due to chloroplast breakdown as indicated by chlorophyll disappearance. The FA composition of the hull lipids resembled that of photosynthetic tissue. In the seeds, the neutral lipids increased from 20% of the total lipids in Phase 1 to 93% at maturity. The proportion of structural lipids declined as the storage lipids increased. In Phase 1 the FA composition of the lipid resembled that of photosynthetic tissue (high in C16:0; C18:2; C18:3). In Phase 2, FA typical of storage triglycerides (C20:1; C22:1, appeared, C18:1 transitorily increased, but C18:2 and C18:3 fell dramatically. In Phase 3, the content of C22:1 continued to rise, but the proportions of the other FA remained constant.