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The effects of increased levels, of dopamine (feeding flies with dopamine precursor, L-dihydroxyphenylalanine) and octopamine (feeding flies with octopamine) on ecdysone 20-monooxygenase activity in young (2 days old) wild type females (the strain wt) of Drosophila virlis have been studied. L-dihydroxyphenylalanine and octopamine feeding increases ecdysone 20-monooxygenase activity by a factor of 1.6 and 1.7, respectively. Ecdysone 20-monooxygenase activity in the young (1 day old) octopamineless females of the strain T beta h(nM18), in females of the strain P845 (precursor of T beta h(nM18) strain) and in wild type females (Canton S) of Drosophila melanogaster have been measured. The absence of octopamine leads to a considerable decrease in the enzyme activity. We have also studied the effects of juvenile hormone application on ecdysone 20-monooxygenase activity in 2-day-old wt females of D. virilis and demonstrated that an increase in juvenile hormone titre leads to an increase in the enzyme activity. We discuss the supposition that ecdysone 20-monooxygenase occupies a key position in the regulation of 20-hydroxyecdysone titre under the conditions that lead to changes in juvenile hormone titre and biogenic amine levels.