1. We have studied the effect of the sensory neuropeptides substance P (SP), neurokinin A (NKA), neurokinin B (NKB) and calcitonin gene-related peptide (CGRP) on microvascular permeability in guinea-pig airways in vivo and investigated whether CGRP would potentiate the effect of SP. We used the extravasation of intravenously-injected Evans blue dye as an index of permeability. 2. The tachykinins SP, NKA and NKB (0.025-5.0 nmol kg-1, i.v.) significantly (P less than 0.05) increased extravasation of dye in a dose-related manner and with a similar pattern of distribution; they were most potent in the trachea and main bronchi, less potent in the larynx and intrapulmonary airways, and had little significant effect in the bladder. 3. SP was significantly more potent in causing extravasation of dye than NKA or NKB with ED50 values (nmol kg-1) in the range 0.04-0.1, depending on the airway level, compared with values in the range 0.3-0.7 for the neurokinins. 4. CGRP (0.0025-2.5 nmol kg-1, i.v.) had no significant effect on microvascular permeability and did not potentiate SP-induced extravasation of dye. 5. Each neuropeptide decreased mean arterial blood pressure, indicating vasodilatation, in a dose-related manner. Co-injection of CGRP and SP produced additive decreases in arterial pressure. 6. We conclude that, in guinea-pig airways, tachykinins increase microvascular permeability via tachykinin receptors of the NK-1 sub-type (indicated by an order of potency of SP greater than NKA = NKB) on endothelial cells. The response appears to be related to mechanisms in addition to vasodilatation. The relevance of the responses to the tachykinins in asthma is discussed.