Experimental viral infections are described which cause malformations of the developing nervous system. These include influenza virus infection of chick embryos causing defects in neural tube closure and flexion, parvovirus infections of rodents and cats resulting in a granuloprival cerebellar malformation, and myxovirus infections of rodents inducing stenosis of the aqueduct of Sylvius. In each experimental model the pathogenesis of the malformation is different, but in each the resultant noninflammatory malformation bears resemblances to malformations in man considered to have a genetic basis.