Euro-Collins (ECS) and UCLA-formula organ preservation solutions induced strong vasocontraction in porcine pulmonary arteries when studied in organ baths at temperatures of 37 degrees C and 30 degrees C. At 20 degrees C ECS induced a 30% contraction, but at 6 degrees C no contraction (n = 5) or a weak contraction (n = 1) was elicited. Neither prostaglandin E1 nor nifedipine caused any significant reduction of the vasocontraction elicited by ECS and UCLA. Krebs solution, enriched with potassium in amounts corresponding to those in ECS (115 mmol/L) or UCLA (30 mmol/L), induced vasocontraction comparing well with those induced by ECS or UCLA, indicating that it is the high potassium content that causes the vasocontraction. In a second experiment lung segments were stored at 4 degrees C for 9 hours in ECS, UCLA, or Krebs solution. Pulmonary arterial segments were then studied in organ baths at 37 degrees C. The choice of preservation solution did not significantly affect the contractile properties of potassium, noradrenaline, or the thromboxane mimic U-46619. To conclude, high potassium contents in organ preservation solutions induce strong pulmonary vasocontraction in lung temperatures greater than 20 degrees C but not in temperatures less than 10 degrees C. These vasocontractions are not significantly reduced by prostaglandin E1 or nifedipine. We suggest that the initial preservation solution used to cool down the lungs should contain 4 mmol/L or no potassium. When the lung temperature is less than 10 degrees C, a second perfusion might be done, and then a high potassium content (if thought to be essential) will not cause vasocontraction.