Fertility study of sodium N-[2-[4-(2,2-dimethylpropionyloxy) phenylsulfonylamino] benzoyl] aminoacetate tetrahydrate (ONO-5046.Na), a novel neutrophil elastase inhibitor, was conducted in Sprague-Dawley (SD) rats. ONO-5046.Na was administered intravenously at doses of 18.75, 37.5 and 75 mg/kg/day to male rats from 64 prior to mating, through the mating period and until necropsy, and to female rats from 15 days prior to mating until Day 7 of gestation, in order to examine its effects on fertility and reproductive performance of males and females and the development of their fetuses. There were no changes attributable to ONO-5046.Na in general signs, body weight, food consumption or autopsy findings in males and females. No drug-related changes were observed in estrous cycles, copulation and fertility indices in males and females. Pituitary weight of dams was decreased in each of the ONO-5046.Na treated groups, but no histopathological changes were observed in the pituitary. In the cesarean section findings in dams, ONO-5046.Na had no effects on the number of corpola lutea, the number of live fetuses, the implantation ratio, the resorbed and dead fetus ratio, fetal or placental weight, or the incidences of external, skeletal or visceral anomalies of the fetuses. From these results, it is considered that the NOAEL of ONO-5046.Na is 75 mg/kg/day for general and reproductive toxicity in males and females and for developmental toxicity in their fetuses.