In vitro studies on E15 rat embryos have shown that the number of receptor neurons containing olfactory marker protein (OMP) is markedly increased when olfactory mucosa (OM) is cultured in direct contact with the presumptive olfactory bulb (POB). This facilitatory influence is tissue-specific; that is, it is absent when the POB is substituted with other nervous or non-nervous tissues. In the present quantitative immunohistochemical study, we show that the enhancing influence of the POB is also a stage-specific phenomenon. The number of OMP-containing receptor cells is greatest when E15 OM is cultured with POB of the same age. If the POB was taken from a less mature (E13) or from a more mature (E17) embryo, the number of OMP-containing cells was greatly reduced.