Darkening of postorbital skin (eyespots) in Anolis carolinensis occurs during stressful situations via adrenal catecholaminergic activation of beta(2)-adrenergic receptors. The eyespots form more rapidly in dominant males during social interaction. It was hypothesized that eyespot darkening from green to black is a social signal that communicates disposition, resulting from sympathetic activation and determines social hierarchy. To assess the value of the eyespot as a signal influencing social dominance, males were paired according to size and weight, and painted postorbitally; one with green and the other with black paint. Painted pairs interacted with aggressive displays, approaches, and bites. Dominant status was determined by continued displacement of an opponent, superior perching position, and lighter color. Males with eyespots darkened by black paint were dominant in 100% of interactions and more aggressive. Males viewing an opponent with eyespots painted black exhibited higher plasma epinephrine (Epi), norepinephrine (NE), and dopamine (DA) levels than males with eyespots hidden with green paint and isolated controls. Results for behavior were similar, but those for plasma catecholamines contrast with those from recent experiments in which interactions occurred vs. a mirrored reflection. The eyespot is a potent stimulus, acting as a social signal that confers dominant status to its bearer, and activates the sympathetic nervous system in opponents that observe it during aggressive interactions.