A decrease in numbers of pulmonary arteries has been implicated in various forms of both human and experimental pulmonary hypertension. However, data in support of this hypothesis have not been conclusive. Especially, intravascular fixation under pressure and the size of the areas of lung tissue subjected to quantitation, may have influenced the results. In order to investigate these factors, we fixed lungs of ten normal rats in three different ways, i.e. by perfusion of the pulmonary vasculature, by immersing collapsed lungs and by instilling the fixative through the trachea with intact thoracic cavity. Subsequently, blood vessels were counted in histological sections. Our study illustrates that perfusion of vessels under pressure decreases the number of recognizable arteries. Also the small areas, used in previous studies for counting vessels, produce unreliable results. Our conclusion is that a decreased number of lung vessels as a factor in pulmonary resistance has not been conclusively demonstrated.