We have examined lungs from adult Wistar rats (n = 6) and four different strains of juvenile and adult mice (n = 40) to localize endothelin-like immunoreactivity. Paraffin sections of lung tissue fixed by distension in Bouin's fluid were stained by the peroxidase-antiperoxidase (PAP) method using 10 different rabbit antisera to endothelin. Immunoreactivity was detected in the majority of epithelial cells of conducting airways from the hilum to the periphery and was similar in rats and all four strains of mice studied. Intense immunostaining was detected in mucous, serous and Clara cells and in occasional alveolar pneumocytes type II. Basal cells and most ciliated cells did not immunostain. From these results it is concluded that endothelin-like immunoreactivity is present in bronchiolar epithelial cells in vivo in rats and mice.