In 12 open-chest pigs, mean weight 18 kg, simultaneous beat-to-beat measurements of pressure in the right and left atria and right and left ventricles, and of pressure and flow in the pulmonary artery, were performed before, during and after the injection of 0.8 ml/kg with 10 to 15 ml/s of diatrizoate, ioxaglate and iopamidol as well as mannitol, normal saline solution and own blood into the right atrium, right ventricle and pulmonary artery. Within 5 beats after injection, all determined hemodynamic values reached their maxima of change independent of site of injection and kind of injectate. After 10 to 20 beats, pulmonary resistance diminished, and the reduction was more pronounced and longer lasting the higher the osmolality of the injectate. Hemodynamic changes during the first beats were a result of the rapidly injected volume; longer lasting hemodynamic changes were associated with an osmolality-dependent decrease in pulmonary resistance.