An immunostaining technique using monoclonal antibodies to a neurofilament protein has allowed us to visualize defects in the development of cranial nerves and ganglia of 10 to 10.5 days mouse embryos following exposure to ethanol in whole embryo culture. Reference patterns for development of cranial nerves and ganglia of control mouse embryos explanted and examined when they had 25 to 34 pairs of somites were established. Additionally, control mouse embryos were grown in whole embryo culture for 48 h, with culture being initiated in embryos having 6 to 7 somite pairs. At the end of the culture period, only minor differences were observed between the control groups. An experimental group of embryos was cultured in the presence of increasing doses (1.6, 3.2, 4, and 4.8 g/l) of ethanol. Defects were observed in the development of the glossopharyngeal and vagus nerves. These abnormalities included absence of the dorsal root (superior ganglion) of IX, star-like shape of inferior ganglion IX, disorganization of the rootlets of nerve X and abnormal fibers between the two nerves and ganglia. These results suggest that the migration and patterning of neural crest cells derived from r6 and r7 may be particularly affected by ethanol. The results also demonstrate the usefulness of this approach in evaluating the susceptibility of the developing cranial nerves to toxicant exposure.