A study of the effect of montirelin hydrate (NS-3), a new drug for the treatment of disturbance of consciousness, during the period of organogenesis was conducted in Sprague-Dawley rats. Female rats were given the drug intravenously at dose levels of 0 (control), 0.02, 1 and 50 mg/kg from day 7 to day 17 of pregnancy. Twenty-three or twenty-five female rats per dose level were sacrificed on day 20 of pregnancy for examination of their fetuses, and the pregnant rats (13-15 per dose levels) were allowed to deliver naturally for postnatal examination of their offspring. In the 1 or 50 mg/kg group, water consumption and the weights of adrenals of the dams increased and the weights of the thymus of the dams decreased. In addition, tremor, disappeared within some minutes, was observed from day 7 to day 16 of pregnancy in all dams given 50 mg/kg. Moreover, food consumption increased and the weights of the submaxillary glands of the dams given 50 mg/kg increased. The drug had no effect on the number of corpora lutea and implantations, on fetal mortality, on fetal body weights, on sex ratio, or on external, visceral and skeletal development of the fetuses. The drug also did not affect delivery. The drug did not have any adverse effects on the newborn such as the number of live newborns, birth index and body weights of live newborn, or on the postnatal development of the first generation offspring (F1) such as differentiation, functional development, emotionality, motor ability, learning ability or reproductive performance. The drug also had no adverse effects of the second generation offspring (F2). These results show that the NOAEL of montirelin hydrate are 0.02 mg/kg for general toxicity in mother animals, 50 mg/kg for pregnancy and delivery of mother animals and 50 mg/kg for development of their offspring.