Emphysema was produced experimentally in rats by administration of porcine pancreatic elastase at dose levels of 75, 100, 125, and 150 units elastase activity/100 g body wt. All doses studied were equally effective in producing emphysema. Dose levels of over 75 units activity/100 g offered no advantage in terms of degree or severity of emphysema produced; however, such doses exacerbated hemorrhage and edema, resulting in relatively high post-instillation mortality. Lesion morphogenesis at all doses tested paralleled those described in this and in other species by other investigators. The 75-unit activity produced a quantifiable degree of emphysema which was relatively constant among animals. Results suggest that because of group differences in susceptibility to elastase (or variations in batches of elastase), dose-range studies should be performed on each new group of animals.