The instillation of elastase into airways is a widely adopted experimental method to quickly produce emphysematous lesions that mimic human disease anatomically and physiologically. Experiments were undertaken to determine whether of not neltenexine, a new drug active on surfactant production, would diminish the severity of this disease. Anaesthetized rats were instilled tracheally with porcine pancreatic elastase (46 U/mg) dissolved in saline, in a single instillation of 0.33 mg/100 microliters. Neltenexine was administered in one experiment at the dose of 25 mg/kg i.p. daily for 30 days. In a second test, neltenexine was given at the same dose six days before the elastase instillation and then by the same schedule as in the first experiment; this was done in search of a possible preventive action. At the end of the treatment, lungs were removed and fixed, and slices were dehydrated, critically point dried, coated with gold and observed by scanning electron microscopy (SEM). Rats that were both pretreated and treated with neltenexine showed a significant reduction in the alveolar deformation induced by elastase. There were no differences between pretreated and treated animals. These experimental findings suggest that neltenexine might prove to be useful for preventing pulmonary emphysema. Biochemical studies in man are needed to confirm the clinical application of neltenexine.