Abstract Heparin was administrated as an aerosol in amounts of 700, 1 300 and 2 000 IU per kg body weight to 16 volunteers on 24 occasions. The concentration of heparin was registered with a chromogenic substrate method. An absorption of heparin via the lung was noted and the concentration of heparin was increased (> 0.02 IU/ml) for 2–3 weeks after 15 inhalations. There was a wide variability in response with regard to concentration of heparin, thrombin time and activated partial thromboplastin time. No dose-response relationship was found. The level of antithrombin and platelet count were mainly unchanged. No untoward effects or changes in pulmonary function were seen.