In the cockroach Blattella germanica, the synthesis of vitellogenin is juvenile hormone III (JH III)-dependent. We have studied the effect of JH III upon vitellogenin gene expression in periovaric fat bodies incubated in vitro. Periovaric fat bodies were obtained from cardioallatectomized females. The response to JH III was measured in terms of vitellogenin and vitellogenin mRNA after 7 h of incubation. A hormonal concentration as low as 1 nM was enough to induce vitellogenin production and its release to the medium, whereas the concentration of 10 nM produced the maximal effects. Although the response of the vitellogenin gene to JH III is fast and efficient, it seems that the action is mediated by protein factors, given that cycloheximide treatment impairs the hormonal effect. The presence in the medium of brain extract (0.5 equivalents), corpora cardiaca (one pair) or hypertrehalosemic hormone (10(-7) or 10(-8) M), partially inhibited the response to JH III.