Previous work has shown that dominant primates in stable social hierarchies in a number of species often have low basal cortisol concentrations, relative to subordinate individuals. In contrast, this trait appears to be lost during periods of social instability, probably reflecting the psychological stressfulness of the instability. The present study examined whether basal cortisol concentrations were elevated not only when the overall hierarchy was unstable, but also were elevated in individuals which, within a stable hierarchy, were nevertheless in the process of their own rank shifting. Study subjects were a population of male olive baboons living freely in a national reserve in East Africa, which could be anesthetized under conditions allowing for determination of basal cortisol concentrations. The instability of a particular rank was quantified by determining the percentage of dominance interactions that represented a reversal of the already established direction of dominance. Highly unstable relationships with the rest of the males in the hierarchy did not predict elevated basal cortisol concentrations in an individual. Instead, it was the stability of interactions with males close in rank (within three steps in the hierarchy of the individual) which predicted cortisol concentrations: the greater the percentage of interactions that were reversals with the three nearest lower-ranking males, the higher the basal cortisol concentrations in an individual. In general, high rates of such reversal interactions indicated that the male was being challenged for his more dominant position.(ABSTRACT TRUNCATED AT 250 WORDS)