The effect of methyl farnesoate (MF) on the ovaries of female red swamp crayfish, Procambarus clarkii, undergoing vitellogenesis was determined both in vivo and in vitro. The in vivo assay showed a positive effect of MF on oocyte growth when injected alone and in combination with 17 beta-estradiol, but not in combination with JHIII or 17 alpha-hydroxyprogesterone. A higher level of incorporation of labeled leucine was induced by MF on isolated pieces of ovary. The same effect was seen when ovary and mandibular organ (MO) were coincubated. These results suggest that MF stimulated the synthesis of vitellin in the ovary of crayfish. In vitro, 17 alpha-hydroxyprogesterone completely suppressed the stimulatory action of the MO on the ovary, suggesting a competitive inhibition between 17 alpha-hydroxyprogesterone and MF on the ovary and/or a negative feedback by that steroid on the MO.