This study reveals the effects of diazepam on the development of neural tube defects in the chick based on light microscopy and histopathological study. Thirty fertile Hubbard Broil eggs were divided into two equal groups. Group 1 embryos (n:15) at stage 8 (the four-somite stage) of development were explanted and grown for 18 hours in nutrient medium (thin albumin). Group 2 embryos (n:15) at stage 8 (four-somite stage) of development wer explanted and grown for 18 hours in nutrient medium containing 400 µg/ ml Diazepam. After the incubation period, 86.6% of the control embryos (Group 1) had intactneural tubes, and 80% of the Group 2 embryos showed neural tube defects. The results of this study suggest that Diazepam causes neural tube defects.