Hamster cheek pouches are bilateral invaginations of the oral mucosa and can easily be everted with their blood flow intact and are, therefore, well suited for intravital microscopy. The preparation has been used extensively for studies of inflammation, tumour growth, vascular smooth muscle function, blood flow regulation, and cellular behaviour at the microcirculatory level. The introduction and use of fluorescein-labelled dextrans (FITC-dextran) as tracers of macromolecular permeability changes have provided evidence that macromolecular permeability is subject to physiological and pharmacological regulation by gap formation between endothelial cells in the postcapillary venules. Studies with clinically used asthma drugs like isoprenaline, terbutaline, budesonide, theophylline and cromoglycate have shown that they all counteract histamine-induced permeability increase in postcapillary venules and, furthermore, that budesonide and terbutaline inhibit increased permeability caused by bradykinin, LTB4 and phorbol-dibutyrate, tertiary butylhydroperoxide and ischaemia. Pharmacological studies on the inhibition of mediator-induced leakage and migration of neutrophils in the cheek pouch could be of value not only for a better understanding of the inflammatory process in general but also for the characterization of asthma drugs having an anti-inflammatory action on the airways.