Studies on growth promoter action can give an insight into the mechanisms of growth control. Calpain genes encode a family of proteins responsible for calcium-dependent proteolytic activity in mammalian cells. The ubiquitous enzymes µ- and m-calpain are activated in vitro by µM and mM calcium ion concentration, respectively. Both these calpain isoforms have been implicated in myofibrillar protein turnover and are regulated by calpastatin, a highly specific inhibitor protein. Overexpression of calpastatin has been strongly associated with skeletal muscle hypertrophy and meat toughness in livestock; a plausible mechanism has been that inhibition of calpain reduces myofibrillar protein degradation while protein synthesis continues unchanged in vivo, whilst inhibition of calpain post mortem prevents proteolytic cleavage of myofibrillar proteins. In pigs there are at least three calpastatin promoters (1xa, 1xb, 1u) with transcription factor binding motifs potentially responsive to β-agonist and IGF-1/calcineurin–mediated signalling pathways (Parr et al., 2001). The objective of the current study is to compare the expression patterns of different calpastatin mRNA transcripts in pigs treated with two different classes of growth promoters: clenbuterol treatment for 24 h and porcine somatotropin (pST) for 7 d.