During craniofacial development in the mouse embryo (days 9–12 of gestation; plug day = day 1), transient expression of serotonin (5-HT) uptake in epithelial structures of this region correlates with critical morphogenetic events (Lauder et al., '88; Shuey, '91; Shuey et al., '89, '92). The purpose of the present investigation was to assess the possible functional significance of these uptake sites by examination of patterns of dysmorphology following exposure of embryos to selective 5-HT uptake inhibitors. Exposure of mouse embryos in whole embryo culture to sertraline, at a concentration (10 μM) which produced no evidence of general embryotoxicity, caused craniofacial malformations consistent with direct action at 5-HT uptake sites. Two other 5-HT uptake inhibitors, fluoxetine and amitriptyline, produced similar defects. The critical period of sertraline exposure occurred on days 10–11. The observed craniofacial defects were associated with decreased proliferation and extensive cell death in mesenchyme located 5–6 cell layers deep from the overlying epithelium. In contrast, the subepithelial mesenchymal layers showed normal or elevated levels of proliferation. From these results it appears that inhibition of 5-HT uptake into craniofacial epithelia may produce developmental defects by interference with serotonergic regulation of epithelial–mesenchymal interactions important for normal craniofacial morphogenesis. © 1992 Wiley-Liss, Inc.