Aerosolized pentamidine was as effective as intravenous pentamidine in treating PCP in animals. More of the pentamidine reached the lungs and less was found in the liver and kidney after pentamidine was given by aerosol than after an intravenous injection. This suggests that the toxicity of pentamidine may be less if given by aerosol than if given by the intravenous route.