Mid-facial development is an extremely complex process involving coordinated events and precise timing. Cleft lip (CL) may result from the failed fusion of the lateral and medial nasal processes in the developing embryo. It has been postulated that spontaneous CL in the A/J strain of mice may be due to a predisposing face shape (Trasler, '68). This hypothesis was examined in mutant mice susceptible to teratogen-induced CL. Mice carrying the dancer (Dc) mutation in the heterozygous state rarely develop CL, whereas 90% of homozygotes (Dc/Dc) develop CL. Outcrossed heterozygotes show elevated susceptibility to 6-aminonicotinamide (6AN)-induced CL (Trasler et al., '84) and these were used to investigate face shape as a predisposing factor. Dc/+ and +/+ males were mated to R stock females, and embryos were collected on day 10/21 hr, when the nasal placodes are approximately at the oblong or crescent stage. Total nasal process areas and volumes, medial and lateral process areas and volumes, and medial jut lengths were measured from histological sections, and comparisons made between the two populations. The results indicate that compared to +/+ control, heads of embryos from the Dc/+ cross have significantly smaller mean total process areas and volumes (P less than 0.005), mean lateral process areas and volumes (P less than 0.005), mean medial process area and volumes (P less than 0.01), mean maximum head diameter (P less than 0.02), but similarly sized medial juts and crown rump lengths. Correlations between maximum head diameter and process size indicate that the Dc mutation may hinder normal development of the nasal processes. These reduced nasal processes may explain the underlying predisposition to 6AN-induced CL.