Pietrain swine homozygous for the hal gene (n) associated with porcine stress syndrome (PSS) and a Pietrain-derivative breed, Near Pietrain (NP), with a frequency of .31 for n, were mated to produce reciprocal F1, F2, and purebred NP litters. The halothane challenge test was performed on all 40 parents and 240 progeny to predict their hal genotype and PSS susceptibility. The DNA-based assay for a C to T mutation at base pair 1,843 of the skeletal muscle ryanodine receptor (ryr1) cDNA, which is very highly correlated with PSS, was also determined for these animals. The predicted hal genotypes observed in the progeny differed significantly from the expected Mendelian ratios, and the halothane challenge test consistently overestimated the n/n hal genotype. However, the ryr1 genotypes observed in the progeny did not differ significantly from the expected Mendelian ratios, and this DNA-based assay apparently misidentified only one of the 40 parents. The results of this study indicate that the assay for the ryr1 mutation more accurately predicts both the homozygous and heterozygous forms of the PSS gene than does the halothane challenge test.