In recent years there has been a considerable reduction in the incidence of some of the more important malformations of the central nervous system as follows: (i) of anencephalus in Birmingham (but not in Scotland); (ii) of spina bifida in Birmingham, England and Wales, and possibly Scotland (although the Scottish data available are inconclusive). We have no explanation for these secular variations, but are satisfied that they are not associated to any considerable extent with fluctuations in the parity composition of the general population of births.