Storage protein synthesis during alfalfa (Medicago saliva L.) seed development displays a temporal pattern and is regulated both by ABA and low osmotic potentials. Deposition of the 2S, 7S and 11S storage proteins occurs maximally at the mid- to late stages of development. Osmoticum, but not ABA alone could effectively induce storage protein synthesis at early stages of development in isolated alfalfa embryos placed on water for several days. Neither ABA nor osmoticum alone could maintain storage protein synthesis in isolated late-stage embryos placed on water. On the other hand, isolated developing embryos cultured on a nutrient (Murashige and Skoog) medium could maintain storage protein synthesis in the presence of ABA and osmoticum, alone or in combination, for up to 14 d after being dissected from the pod. Of the components of this medium, the inorganic salts appeared to be the most important. The response to ABA and osmoticum varied with the time of isolation during development with the greatest enhancement of storage protein synthesis in isolated embryos coinciding with the time of maximum synthesis within the seed when in planta. Addition of ABA and osmoticum after the time of maximum storage protein synthesis did not elevate the amount of synthesis, but rather prolonged the time over which it could take place. Thus, while the amplitude of storage protein synthesis could be modified by ABA or osmoticum, the inherent temporal pattern, with maximum synthesis occurring only at mid- to late stages of embryo development, could not