The calpain proteolytic system was examined in the longissimus muscle (LD) of heterozygote pigs carrying a single copy of a mutation in the skeletal muscle ryanodine receptor gene (RyR1) that is associated with porcine stress syndrome and reduced meat quality. Conventional British White-type pigs (n = 30) were selected from a commercial line on the basis of slaughter weight, backfat depth, and pH at 45 min postmortem > 6.0; based on DNA analysis, 11 were heterozygous RyR1 mutants (Nn), and 19 were normal genotype (NN). The LD samples were taken from carcasses at 2, 4, and 24 h postmortem for calpain analysis with enzyme assay and immunoblotting, using specific antisera raised against recombinant polypeptides derived from calpain large subunits and calpastatin. Shear force (SF) was measured after conditioning for 8 d at 2 degrees C and did not differ between Nn and NN groups. The extractable activity of mu-calpain decreased over 24 h postmortem (P < .001), with no significant difference in activity between NN and Nn animals at any time. The activity of m-calpain also decreased with time (P < .001), but it was lower at all times in Nn than in normal genotypes (P < .001). After Western blotting, the immunoreactivity of mu- and m-calpain large subunit bands declined over 24 h postmortem (P < .001); values for mu-calpain were higher (P < .05) and for m-calpain were lower (P < .001) in heterozygotes than in normal animals at each sampling time. The calpastatin antibody detected a major band of 135 kDa that declined with time postmortem but did not differ between Nn and NN genotypes at any sampling time. These data indicate that the levels of extractable mu- and m-calpain, but not calpastatin, may be different in pigs that carry the RyR1 mutation.