The induction of kidney tubules in metanephric mesenchyme has previously been shown to require close contact between the interacting tissues. In our study we show that low concentrations of inhibitors of RNA, DNA and protein synthesis inhibit tubule induction, although they do not seem to prevent the formation of contacts between the interacting tissues. The effective concentrations were about the same as those which inhibited the synthesis of macromolecules. Cycloheximide caused an increased synthesis of RNA. Low concentrations of Mitomycin C inhibited DNA synthesis but not tubule formation. A concentration of the inhibitors which caused weakened induction also caused a marked decrease in leucine incorporation. We concluded therefore, that a decrease in protein synthesis in the inducing tissue is responsible for the inhibition of induction.