Oestrogens, a family of signalling hormones, are important for establishing sexual dimorphism during organogenesis. The role of oestrogens before sex differentiation, however, is less well understood. Now, Aya Takesono, Charles Tyler and colleagues employ a zebrafish oestrogen biosensor, together with chemical and genetic cell ablation techniques, to reveal the function of oestrogens during embryonic brain development. The authors show that cells in the olfactory bulb, termed EROBs, primarily respond to induced oestrogen signalling. EROBs increase in number throughout embryonic and larval development and express the astrocyte glial marker GFAP and the oestrogen-synthesising enzyme aromatase B. EROB projections interact with olfactory sensory neurons in olfactory glomeruli and EROB ablation affects glomeruli location and size. In addition, chemical inhibition of oestrogen signalling reduces the number of inhibitory synapses formed, indicating that oestrogen/EROBs are also required for establishing inhibitory synaptogenesis in the olfactory glomeruli. By imaging calcium sensor-expressing and EROB-deficient zebrafish, the researchers reveal that activation or inhibition of oestrogen signalling reduces or increases neuronal activity, respectively, in the olfactory bulb through EROBs. Finally, both inhibition and activation of oestrogen signalling affect odorant-response behaviour. This study provides evidence that oestrogens act on EROB glia to regulate olfactory circuits.