Although essential for normal seed development in the legumes, the metabolic role of the endosperm remains uncertain. We designed noninvasive nuclear magnetic resonance tools for the in vivo study of key metabolites in the transient liquid endosperm of intact pea (Pisum sativum) seeds. The steady-state levels of sucrose, glutamine, and alanine could be monitored and their distribution within the embryo sac visualized. Seed structure was digitalized as a three-dimensional model, providing volume information for distinct seed organs. The nuclear magnetic resonance method, combined with laser microdissection, isotope labeling, in situ hybridization, and electron microscopy, was used to contrast the wild-type endosperm with that of a mutant in which embryo growth is retarded. Expression of sequences encoding amino acid and sucrose transporters was up-regulated earlier in the endosperm than in the embryo, and this activity led to the accumulation of soluble metabolites in the endosperm vacuole. The endosperm provides a temporary source of nutrition, permits space for embryo growth, and acts as a buffer between the maternal organism and its offspring. The concentration of sucrose in the endosperm vacuole is developmentally controlled, while the total amount accumulated depends on the growth of the embryo. The endosperm concentration of glutamine is a limiting factor for protein storage. The properties of the endosperm ensure that the young embryo develops within a homeostatic environment, necessary to sustain embryogenesis. We argue for a degree of metabolite-mediated control exerted by the endosperm on the growth of, and assimilate storage by, the embryo.