A mutation in the gene coding for the ryanodine receptor 1 (RYR1), also known as halothane (hal) gene or swine stress gene, is associated to the porcine stress syndrome (PSS). Detection of the mutation is normally accomplished by PCR amplification of an 81bp fragment of the hal gene, followed by digestion with the HhaI restriction endonuclease. Wild-type allele (N) is cut in two fragments, whereas the mutant allele (n) is not digested by the restriction enzyme. Electrophoresis of the digested DNA on agarose gel and ethidium bromide staining allows the reading of the result. The correct interpretation is difficult due to the small size of the DNA fragments. In this study we designed a new set of primers for amplification of a 144bp fragment that facilitates the reading of the result. In addition, we optimized the PCR reaction to allow amplification from a single hair bulb, added directly into the PCR mix without previous treatment. This improved method was used to genotype 165 sows and boars used in a breeding program. Forty-nine percent of the animals had the NN genotype, whereas 50 were Nn and only 1 was nn.