Using a left lung orthotopic isograft model in adult male AS strain rats, the pathology of lungs which were stored for 48 or 72 hours using a simple organ flush technique followed by low temperature (0 degrees C) immersion has been investigated. Lungs were examined after cold storage alone and after storage followed by either brief (up to 1 hour) or extended (30 days) reperfusion with blood in vivo. Grafts were flushed with either isotonic saline (NaCl) or hypertonic citrate solution (HCA) alone, or with HCA containing either verapamil (a Ca(2+)-channel blocker) or prostacyclin (PGI2 which has both anti-platelet and vasodilator actions). Controls included fresh non-flushed lungs and fresh HCA-flushed lungs which were transplanted immediately after harvest. After prolonged (48 or 72-hour) cold ischaemia alone the only clear change in lung morphology was of nuclear swelling. Early reperfusion changes included: (i) oedema (interstitial and alveolar); (ii) vascular congestion; and (iii) intra-alveolar haemorrhage. Features (i) and (ii) were diffuse whilst haemorrhage was patchy. Lungs which remained in vivo for up to 30 days showed focal scarring and chronic inflammation with numerous macrophages containing haemosiderin; the extent of the changes observed in individual grafts tended to mirror the gross macroscopic outcome. Attempts to improve the cold ischaemic tolerance with added agents (verapamil and prostacyclin PGI2) failed to produce a clear advantage.