Homozygotes for the splotch (Sp) mutation in the mouse have spina bifida, whereas the heterozygotes have a white belly spot but otherwise appear normal. Spina bifida can be induced by maternal treatment with retinoic acid. Female SWV strain mice were treated intraperitoneally with retinoic acid suspended in peanut oil 8 days/12 hours after they had been mated to either Sp/+ or +/+ males. Probit analysis of the dose-response data suggests that the presence of the Sp gene causes an increased susceptibility of the embryo to the spina bifida-causing effects of retinoic acid. To study the nature of this increase litters were obtained on gestation day 9 from untreated SWV females mated as above. The mean length of the posterior neuropore (the length of the posterior neural tube that has not yet closed) was determined for each somite number between 14 and 26 and was found to be significantly greater in embryos from the Sp/+ cross. This delay of closure of the neural tube in Sp/+ cross embryos could explain the observed increase in their susceptibility to retinoic acid.