Plasma corticosterone has been correlated with the susceptibility to animal hypnosis in male rabbits. Subjects (Ss) were considered susceptible when mean hypnosis duration was above 30 sec or when initial duration (duration of trial 1) was greater than 0. A positive significant correlation was found between mean and initial duration. A significant difference was found between corticosterone levels in susceptible and unsusceptible Ss: higher values were associated with susceptible SS and with SS with initial duration greater than 0. These results indicate a higher susceptibility to animal hypnosis in rabbits with a more active adrenocortical system.