RESUMO
Fifty rats aged four days were divided into five groups: Group I was the control group, groups II-V animals were given intraperitoneal methotrexate injection with escalating doses [2.1, 3.5, 8.4 and 14 mug/g body weight]. The animals were observed for gross changes and were decapitated at two weeks after injection. Skull radiographs were taken and tooth germs were dissected and submitted for histological study. All rats of groups IV and V died in less than one week with growth arrest. 40% of group III died within two weeks. The LD50 = 4.9 mug/g body weight and LD10 = 1.9 mug/g body weight. Reduction in the average weight of groups II and III rats compared with the control group was observed, which was dose dependent [F = 112.6882, P <0.0001]. Radiographically and histologically, there was delay in dental eruption with disturbances of odontogenesis that were dose related. Methotrexate in non-lethal doses causes a dose-related reduction in growth and dental development. The proliferating undifferentiated mesenchymal cells are more sensitive to methotrexate than odontogenic epithelium