Adult males of the small arboreal iguanid lizard, Anolis carolinensis, form social dominance hierarchies when placed in habitats with limited resources. Skin color changes occur during hierarchy formation, most conspicuously in subordinates, who appear darker (more brown) than dominants (more green). Because skin color in this species is under the control of hormones frequently associated with physiological stress, radioimmunoassay of plasma levels of the principal reptilian adrenal steroid, corticosterone, was performed. To examine the influence of gonadal androgen, known to influence the aggression that attends hierarchy formation, lizard pairs were constituted in which one or both members were castrated. Corticosterone levels of intact subordinates were significantly elevated, whereas those of castrated subordinates or dominants showed levels comparable to those of isolates. No significant differences in spermatogenic stage could be detected between intact dominants or subordinates.