The study aims to show that ondansetron, which is used safely in pregnant women, can cause serious side effects. Neural tube defects are among the most common congenital malformations of the central nervous system. It is known that genetic predisposition, environmental factors, and some drugs play an important role in the development of neural tube defects. Ondansetron is a selective 5-hydroxytryptamine-3 receptor antagonist used in the treatment of cancer, nausea, and vomiting during pregnancy and after anesthesia. In the literature studies, it was not found that developmental anomalies were observed. Seventy-five free specific pathogen eggs were incubated for 32 hours and divided into five groups of 15 eggs each, including a control group. Ondansetron was administered to these five groups by sub-blastoderm route in 4 different doses with a Hamilton microinjector. At 48 hours of incubation, the embryos were dissected and examined morphologically and histopathologically. At the end of the study, a significant dose-dependent decrease was observed in crown-rump lengths, somite numbers, and mean the number of silver-dyed nucleolar regulatory regions (AgNOR) and total AgNOR / nuclear area ratios. Statistically significant differences were observed between the experimental groups in terms of neural tube closure (p
