Abstract Vitellogenin synthesis can be induced in vivo by oestrogen in the livers of Xenopus laevis larvae during metamorphic climax. The timing of the appearance of vitellogenin inducibility has been precisely defined in relation to changing morphological characteristics, and occurs during normal development at stage 62. This suggests that thyroid hormones may be involved in the transition from the uninducible to the inducible state of the vitellogenin gene. Chemical inhibition of the larval thyroid by propyl thiouracil or thiourea was used to produce developmentally static larvae. Tests on these animals for response to oestrogen showed that the transition depends on metamorphic stage, not on chronological age. Immersion of normal tadpoles or developmentally static tadpoles in high concentrations of thyroxine induces rapid metamorphosis, but does not alter the developmental stage at which vitellogenin inducibility is acquired in either case. Induction of vitellogenin synthesis in animals before stage 62 was only achieved by permitting thyrostatic animals to continue a delayed metamorphosis. It is apparent that thyroxine does not directly confer inducibility on the vitellogenin gene.