Plasma cortisol levels were measured in Landrace pigs susceptible to malignant hyperthermia (MH), Landrace pigs resistant to MH and in Large White pigs, during growth, halothane anesthesia and after injections of Synachten. From 3-6 mo. of age mean cortisol levels in susceptible Landrace were 193.2 .+-. 49.7 nmol/l, in resistant Landrace 220.8 .+-. 24.8 nmol/l and in Large White 278.8 .+-. 13.8 nmol/l. These values were not significantly different. After injection of 1 mg Synachten i.v., plasma cortisol levels did not increase significantly in susceptible pigs. In resistant Landrace pigs and Large White pigs the plasma levels after injection were significantly greater than pre-injection levels (P < 0.05) and significantly different from susceptible Landrace levels. During halothane anesthesia, cortisol levels rose more in resistant pigs than in susceptible pigs but differences were not significant. Susceptible pigs had a decreased ability to secrete cortisol during stress.