This paper recounts a study of the prevalence of malignant hyperthermia in a population of boars entering the Record of Performance Test Station at New Hamburg, Ontario over a period of ten months. The literature is briefly reviewed and an account of the present status of malignant hyperthermia (porcine stress syndrome) in Ontario boars using the halothane or halothane/succinylcholine screening tests is presented. It was determined in this population of boars that there was a 1.5% prevalence rate of malignant hyperthermia susceptible boars. This was based on a five minute halothane challenge carried out on 786 boars from 107 herds. The halothane reactors came from a total of eight herds or 7.5% of the total number of herds. A majority of 58.3% of the animals reacted after the three minute mark of the halothane challenge. In contrast, an 18.0% prevalence rate was determined using halothane/succinylcholine challenge on 123 boars. This further identified an additional eight herds with the problem, bringing the total to 16 or 15% of the total number of herds. It is suggested that the prevalence of malignant hyperthermia in Ontario breeding herds is much higher than was originally thought and that the halothane challenge is an inadequate screening test for this trait if the intention is to remove the genetic trait from the breeding herd. At the same time halothane challenge testing will identify those animals which have a strong susceptibility to the trait and if used on key breeding stock will help to maintain some control on the condition until a more definitive test is readily available.(ABSTRACT TRUNCATED AT 250 WORDS)