Thiopentone was given to eight women and eight men (60 - 70 yr). The disappearance of thiopentone from the venous blood was described by a three-compartment open model. The only significant difference between the sexes was a higher initial venous concentration in males. The dose (mg kg-1) for induction was 70% of the value (P less than 0.05) previously reported for a comparable group of younger men and women (20 - 40 yr). The volume of distribution V2 and V2 were larger in the elderly (P less than 0.05). The terminal half-lives were increased with advancing age (from 75% to 100% on average) (P less than or equal to 0.01). The clearance value was 50% greater in the older women than in a group of young women. For all groups a significant correlation between initial drug concentration and k12 supported th hypothesis that there distribution rate constant k12 is the predominant factor in the pharmacokinetic profile of a dose of thiopentone sufficient to obtund the eyelash reflex.