The effect of dietary supplementation with clenbuterol for either 8 days or 55 days in lambs was studied. The 55-day treatment was combined with an immediate preslaughter exercise regime. The effect of these treatments on post-mortem calpain system activities, meat ageing and meat quality was studied. Neither short-nor long-term supplementation had an effect on the rate of pH fall post mortem. Short-term supplementation had no effect on the initial nor the final shear force values but these were higher at intermediate times. In contrast, prolonged supplementation increased shear force values at all times post mortem. Preslaughter exercise, while influencing the rate of pH decrease in both control and supplemented groups, did not affect meat tenderness. After short-term clenbuterol-supplementation, the in-vitro μ-calpain activity was significantly lower in the supplemented group at 6 and 24 hr post mortem, while m-calpain and calpastatin activities were largely unaffected. In contrast, 55-day clenbuterol supplementation resulted in significantly higher levels of calpastatin activity at all times post mortem. These data imply that clenbuterol results in toughened meat due to alterations in the calpain/calpastatin system, the mechanisms of which are dependent upon the duration of supplementation.