Neuropeptides in the lung occur in neurons, neuroendocrine and inflammatory cells. Their widespread distribution and physiological effects suggest that they may play important roles in asthma. We investigated whether, during an exacerbation of asthma, patients displayed changes in plasma levels of the neuropeptides vasoactive intestinal peptide (VIP), calcitonin gene-related peptide (CGRP), substance P (SP), and neuropeptide Y (NPY). Venous blood from 25 adult patients attending the emergency ward with an exacerbation of asthma was sampled before and after treatment. Plasma levels of VIP-, SP-, CGRP- and NPY-like immunoreactivity (-LI) were determined by immunoassay, and the results obtained were compared with findings in 21 healthy controls. The mean plasma levels of VIP-LI were lower in patients (3.4 +/- 0.4 pmol.l-1) than in controls (10.4 +/- 0.7 pmol.l-1), whereas the levels of CGRP-LI (43.7 +/- 3.4 pmol.l-1), SP-LI (4.6 +/- 0.4 pmol.l-1) and NPY-LI levels (159 +/- 6 pmol.l-1) were higher in patients than in controls (21.1 +/- 3.4; 2.2 +/- 0.2 and 105 +/- 8 pmol.l-1, respectively). A relationship was seen between the reversibility of obstruction, expressed as improvement of peak expiratory flow upon treatment, and the neuropeptide levels, such that lower VIP-LI levels and higher CGRP-LI levels correlated with less reversibility. Plasma levels of neuropeptides, VIP-LI and CGRP-LI in particular, may therefore be employed as predictors of responsiveness to bronchodilatory therapy.