Pea (Pisum sativum L.) cotyledons, overexpressing a potato sucrose transporter (StSUT1), were used to explore the hypothesis that sucrose stimulates the onset of storage protein biosynthesis. The study focused on the transition between pre-storage and storage phases of seed development. During this period supply of sucrose and hexose to transgenic cotyledons was unaffected by StSUT1 expression. However, protoplasmic levels of sucrose but not hexoses were elevated in transgenic cotyledons. Total protein levels in cotyledons followed the same temporal trend as observed for sucrose and this was reflected in an earlier appearance of protein bodies. Protein levels in wild type and StSUT1 cotyledons were found to lie on the same sucrose dose-response curve and this could be reproduced in vitro when wild type cotyledons were cultured on media containing various sucrose concentrations. Rates of [¹⁴C]sucrose uptake and incorporation into polymeric forms were consistent with protoplasmic sucrose supplying a proportion of the carbon skeletons required for storage protein accumulation. In addition, vicilin gene expression was up-regulated earlier in StSUT1 cotyledons. We conclude that sucrose functions both as a signal and fuel to stimulate storage protein accumulation and assembly into protein bodies. An earlier stimulation of storage protein synthesis is considered to largely account for the 14% increase in protein levels of StSUT1 seeds at harvest.