Samples were obtained from the Longissimus lumborum muscle of 5 halothane-negative and 7 halothane-positive pigs as follows: 1, just before slaughter a biopsy was performed and half of the biopsy sample (sample 1) was immediately used for analysis; 2, the other half of the biopsy (sample 2) was incubated at 39 deg C with 0.5 ml KCL (150 mM) for 1 h before use; 3, muscle was cut from the carcass 1 h after slaughter (sample 3) and 24 hours after slaughter (sample 4). Myofibrillar spacing and extracellular space were determined by histological techniques in all samples, while pH and water holding capacity (WHC) were determined in samples 2 and 3. The changes in pH, WHC, myofibrillar spacing and extracellular space appeared roughly parallel in post mortem muscle. All these traits had almost reached their ultimate values 1 h after slaughter. Myofibrillar spacing and extracellular space had undergone little change in biopsy after 1 h incubation although pH was much decreased. Stress associated with slaughter could play a prominent role in the post mortem changes of myofibrillar spacing and extracellular space in Longissimus lumborum muscle