We have recently established an autism model rat by exposing embryonic day (E) 9 rat to known autism-inducible teratogen, thalidomide (THAL) or valproic acid (VPA). They showed;1)hyperserotonemia and significant increase of serotonin concentration in the hippocampus (Narita et.al.,Pediatr Res 2002),2)dramatic shift of the distribution of serotonergic neuron in the dorsal raphe nuclei, the origin of serotonergic neurons (Miyazaki et.al.,Int J Dev Neurosci, in press),3)behaviral defects in learning and memory examinations (Miyahara et. al., SFN meeting 2003). Since these phenotypes are, to some extent, relevant to human autistic phenotype, we hypothesized that disorganized serotonergic neuronal development commonly triggered by the teratogens in the model rats may play a key role in autism. We aimed to observe the effects of the E9-exposed teratogens on later serotonergic neuronal differentiation and migration. THAL, VPA, or vehicle was orally administered to pregnant rats on E9, and the embryos were dissect...