The olfactory system is peculiar in the strategy developed to identify a large array of possible stimuli. Each olfactory receptor neuron synthesizes one or a small number of odorant receptor molecules chosen from a large repertoire.To understand when and how this choice is made we cloned several odorant receptor genes in the chick (Leibovici et al, here after). We used an in situ hybridization technique to study their expression during embryonic development. We noticed that the activation of the genes was precocious in cells originating from the olfactory placode exclusively. However the expression pattern was modified at the beginning of the second week of incubation. The number of genes expressed by any single neuron was restricted, with an increase of mRNA level per cell. Expression was subsequently confined to olfactory neurons without specific regional, epithelial localisation. This transition period coincided with the beginning of synaptogenesis between sensory neurons and mitral cells in the bulb. Further studies of odorant receptor gene expression in experimental conditions tended to confirm the importance of bulbo-epithelial interactions for the expression and eventually the choice of odorant receptors by the olfactory neurons.