It has been suggested that ruthenium red has a selective inhibitory effect on capsaicin-induced nociceptor stimulation. We have investigated the effect of ruthenium red on oedema formation and vasodilatation induced by intradermal (i.d.) injection of capsaicin in the rabbit in vivo. Responses induced by capsaicin were inhibited by ruthenium red, but responses induced by bradykinin, N-formyl-methionyl-leucyl-phenylalanine (FMLP), platelet activating factor (PAF), histamine and calcitonin gene-related peptide (CGRP) were not affected. These results suggest that ruthenium red selectively inhibits capsaicin-induced local plasma protein leakage and vasodilatation in the rabbit skin microvasculature.