We modified the double capillary method of Philipoff et al. [23] to allow rheologic measurements of lower respiratory secretions. The system consists of a precision bore stainless steel capillary (0.58 mm i.d., 5 mm length), a glass capillary (0.92 mm i.d., 15 mm length) and a sample holding tube (1.5 mm i.d., 15 mm length). This series of capillaries is connected to a long polyethylene catheter which permits advancing the system through the inner channel of a fiberoptic bronchoscope, and the placing of the sample holding tube into the lower airways for the aspiration of secretions under direct vision. The required sample volume is 20 microliter. The capillary is then removed from the fiberoptic bronchoscope and placed on a microscope stage; the movement of the meniscus of the sample under an applied vacuum as well as the elastic recoil of the sample after release of the vacuum are measured at body temperature. From this procedure, apparent viscosity (na) and shear elastic modulus (G) are calculated. The accuracy of the method was verified in vitro by using standard viscosity oils and a polymer solution of known elasticity. In five normal conscious sheep, na ranged between 200-3000 poises (shear rate: 0.06-1.0 s-1) and G between 17-83 dyn X cm-2. The micro double capillary method was found adequate for direct rheologic measurements of lower airway secretions.