Intracorporal, especially pulmonary, and extracorporal deposition of pentamidine was studied following inhalation of 300 or 600 mg in 6 ml Aqua dest. each dose in six patients with one previous episode of pneumocystis carinii pneumonia. Three different nebulizers--two mechanical nebulizers (Respirgard II and Pari-IS-2) and one ultrasonic device (Portasonic) - were compared. The following results were obtained: 1.300 mg pentamidine in 6 ml Aqua dest. is a sufficient dose for prophylaxis provided an appropriately constructed nebulizer and an optimal respiratory manoeuvre (inspiratory vital capacity breathing) are employed. With the nebulizer Portasonic, the frequency of cough was higher compared with the two mechanical nebulizers, which is attributed to its higher mist density. 2. Application of 600 mg pentamidine in 6 ml Aqua dest. as an aerosol is difficult to apply owing to clinical and technical problems caused by severe respiratory tract irritation and a high viscosity of the nebulizer solution. We assume that ultrasonic nebulization is affected more by the high viscosity than mechanical pneumatic nebulization. 3. Inspiratory vital capacity breathing (12/min) leads to five- to eight-fold higher pulmonary and improved peripheral deposition rates compared with commonly used spontaneous respiration. We therefore recommend to shorten the duration of inhalation by applying this respiratory technique while maintaining a generally tolerated and optimally nebulizable concentration of pentamidine, i.e. about 300 mg in 6 ml Aqua dest.