Abstract Objective—To evaluate the effect of homologous amniotic fluid and meconium inoculated intratracheally into the lungs of neonatal rats. Animals—153 male 7-day-old Fischer-344 rats. Procedure—Amniotic fluid was obtained by cesarean section from the uterus of pregnant rats and meconium was collected at the time of birth from the gastrointestinal tract of neonatal rats. Neonatal rats were randomly allocated into 5 treatment groups. Two groups received 0.05 ml of saline (0.9% NaCl) solution; the third and fourth groups received 0.05 ml of 50% or 100% amniotic fluid, respectively; the fifth group was inoculated with 0.05 ml of a 20% suspension of meconium. Six or 7 rat pups/group were euthanatized by exsanguination under halothane anesthesia at postinoculation days 1, 3, 7, and 14. The magnitude of injury and inflammatory response was determined by biochemical and cytologic analyses of bronchoalveolar lavage fluid. Results—Inoculation with saline solution and amniotic fluid did not induce pulmonary injury or inflammatory response. Inoculation with meconium induced significant ( P < 0.01) injury and inflammatory response, characterized by the release of cytosolic enzymes and recruitment of neutrophils in the lung. Conclusion and Clinical Relevance—Saline solution is an innocuous vehicle that can be safely used in intratracheal inoculations in neonatal rats. Homologous amniotic fluid, despite containing keratin and epidermal cells, does not cause acute injury or inflammation in the lung. In contrast, meconium acts as a toxic substance injuring respiratory cells and causing a vigorous but transient leukocytic inflammatory reaction in the lungs. (Am J Vet Res 2001;62:1636–1641)