Previous studies using an antibody to cis-urocanic acid and mast-cell-depleted mice implicated both cis-urocanic acid and mast cells in the mechanisms by which ultraviolet B light suppresses systemic contact hypersensitivity responses in mice. In the absence of a direct stimulatory effect of cis-urocanic acid on connective tissue mast cells, an indirect association was investigated. A blister induced in the rat hind footpad was used to examine the effects of slowly perfused cis-urocanic acid on cutaneous blood flow. cis-Urocanic acid but not trans-urocanic acid increased microvascular flow by a mechanism largely dependent on the combined activity of the neuropeptides, substance P and calcitonin gene-related peptide. Perfusion of cis-urocanic acid over the base of blisters induced in sensory-neuropeptide-depleted rats did not have any stimulatory effect above that seen with perfusion of cis-urocanic acid together with neuropeptide receptor antagonists in control rats. There was a small direct effect of cis-urocanic acid on microvascular blood flow. As both substance P and calcitonin gene-related peptide could directly degranulate connective tissue mast cells, this study suggests that cis-urocanic acid indirectly activates mast cells via its effects on peripheral terminals of unmyelinated primary afferent sensory nerves. cis-Urocanic-acid-induced neuropeptides may also contribute to ultraviolet-B-induced cutaneous inflammation and alterations to Langerhans cell activity.