Exogenous mevalonate poorly stimulated juvenile hormone III (JH-3) biosynthesis by corpora allata from Locusta migratoria. However, mevalonolactone strongly stimulated glands from different physiological states and fully restored JH synthesis in mevinolin-treated glands. Asymmetry in spontaneous rate of JH release was abolished by exogenous mevalonolactone. After transection of nervus corporis allati 1 (NCA-1), the rate of mevalonolactone-stimulated JH synthesis was maintained at the preoperative levels although the spontaneous rate of JH biosynthesis decreased rapidly. These results suggest that the spontaneous asymmetry of JH biosynthesis and the low rate of JH biosynthesis by denervated corpora allata both result from non-stimulation or inhibition acting on the JH pathway before the utilisation of mevalonate.