A range of ultimate pH (pH(u)) values, from 5.4 to 7.2, was produced in the m. longissimus thoracicum et lumborum (LD) of 55 sheep by subcutaneous injections of adrenaline, or by injection and subsequent exercise. The rate constant of ageing for each of 47 animals was calculated from shear force measurements, taken at intervals for up to 5 days from the LD held at 12 dgC. The relationship between the ageing rate and pH(u) could be adequately described by the equation rate constant (k) = 4.109 - 1.349(pH) + 0.1111(pH)(2); r(2) = 0.707, with a minimum rate constant at pH 6.07. The loin muscles from eight carcasses were selected to determine the effects of ultimate pH on the initial, unaged toughness value. Calpain activity was inhibited by intramuscular injection of ZnCl(2) dissolved in saline, and the muscles were aged at 10-12 °C on the carcass for up to 6 days. At all values of pH(u) (5.4-6.7), tenderness as measured by shear force were equivalent, from which it was concluded that the initial toughness of unaged lamb was not affected by pH(u). In the contralateral, untreated muscle, ageing was measured using both shear force measurement and changes in the myofibrillar fragmentation index (MFI). After 1 day, the intermediate pH range (5.8-6.2) has the highest shear force and the lowest fragmentation index value. However, in contrast to the shear force values, the ultimate MFI value in aged muscle was lowest in the high pH(u) range (6.4-6.7; minimum value at 6.4). The highest fragmentation values were found at the low pH(u) range. These observations suggest that pH(u)-related tenderness differences in meat result from different rates of ageing in the post rigor period, and therefore that these differences disappear following a sufficient ageing period. The slowest rate of tenderization measured on cooked meat was observed at pH(u) values around 6.07, but the least fragmentation of raw meat was observed at pH(u) values around 6.4, a difference of almost 0.33 pH unit.