We studied whether cortical cells migrate and subplate cells are lost in explants of murine cortex cultured for up to 14 days in defined serum-free medium. We gave bromodeoxyuridine (BrdU) to mice on embryonic days 12-13 (E12-13), to label subplate cells, or E17, to label cells destined for superficial cortical layers. We started culturing on E18-20. We left some mice to develop and provide in vivo data. At E18-20, most cells born on E17 lay below the cortical plate. During the following days in vitro, many of them moved to the cortical plate's superficial edge, although an abnormally high number remained in the ventricular zone. There was a selective loss of BrdU labelled cells from the subplate in culture; this occurred more rapidly than in vivo. Factors absent from our cultures may normally enhance migration and prolong the survival of subplate cells.