The antiepileptic drug valproic acid (VPA) has been implicated as a human teratogen causing spina bifida aperta. Recently we developed a mouse model inducing spina bifida aperta and occulta with VPA. In a search for novel antiepileptic agents the VPA metabolite E-2-n-propyl-2-pentenoic acid (2-en-VPA) had been developed. In the mouse, 2-en-VPA exhibits anticonvulsive potency similar to VPA but very low teratogenic potency (induction of exencephaly). We have now compared VPA and its metabolite 2-en-VPA in regard to induction of spina bifida in our mouse model. 2-en-VPA was administered 3 times during the period of gestation most sensitive for the induction of spina bifida aperta with VPA: on day 9 of gestation at 0, 6 and 12 h. The following doses were injected (in mmol 2-en-VPA-Na/kg): (a) 3 x 2.1, (b) 3 x 2.7 (the equimolar dose of VPA is the threshold dose for induction of spina bifida aperta) and (c) 3 x 3.0 (the equimolar VPA dose produced spina bifida aperta). 2-en-VPA did not induce spina bifida aperta in the mouse in any of these groups. We then investigated the induction of spina bifida occulta in the three dose groups. Spina bifida occulta is a less serious form of spina bifida and may provide a sensitive method to estimate the potency of a compound to induce more severe forms of spina bifida. This malformation was demonstrated in alcian-blue- and alizarin-red-stained fetal skeletons by measurements of the distance between the cartilaginous ends of each vertebral arch.(ABSTRACT TRUNCATED AT 250 WORDS)