After [3 H] thymidine injection to new-hatching Drosophila melanogaster females the migration of labelled egg-chambers through the ovarioles was studied by autoradiographies of the ovaries at daily intervals. The growth duration of egg-chambers which progressed during the first 48 hr was 24 hr longer than the growth duration of chambers progressing thereafter. This temporary delay of ovarian activity was related to endocrine modifications occuring in female during this s period and particularly to a JH-ecdysone interaction.