Reproduction in female lobster (Homarus americanus) is characterized by the maturation of the ovary, with a gradual increase in its size as a result of uptake of yolk protein precursor, vitellogenin (Vg) to the final product vitellin (Vn). Vn is formed by aggregation of several Vg subunits. In most decapods, the hepatopancreas is the major site of vitellogenin biosynthesis. The production of vitellogenin is controlled by endocrine factors. In this study, the effect of farnesoic acid (FA) and 20-hydroxyecdysone (20E) on production of vitellogenin by hepatopancreas (HaVg1) was investigated by in vitro organ explant HaVg1 gene expression was stimulated by FA or 20E in a dose-dependent manner. A 2-fold and 2.2-fold increase in HaVg1 gene expression was observed with 4.2 microM FA and 0.7 microM 20E, respectively. The stimulatory effect by either FA or 20E was observed principally during the first 90 min. Stimulation of HaVg1 gene expression by FA and 20E together is greater (3.3-fold increase) than that of either hormone alone. This stimulation was also observed within the first 90 min. To study the synergistic effect of these two hormones, FA and 20E were tested separately and together at low concentration (42.3 nM and 6.7 nM, respectively). Combined use of FA and 20E increased HaVg1 gene expression synergistically, but not additively. These findings should contribute to our understanding of lobster reproduction and provide insights into manipulation of lobster reproduction in aquaculture or under captive conditions. (c) 2009 Elsevier Inc. All rights reserved.