It has been known for several years that small airways of the lung (<2 mm internal diameter) may be defective in the absence of either clinical symptoms or abnormal function tests such as Forced Expiratory Volume in one second (FEV1), Forced Vital Capacity (FVC) or Peak Expiratory Flow Rate (PEFR). Interest in this "silent zone" has led to the development of special tests of small airway function. Some of these special tests including Frequency Dependence of Compliance and Deposition Patterns using radionuclide tagged particles are not readily adaptable to epidemiological screening. Others such as measurements of instantaneous flow rates on Maximal and Partial Expiratory Flow Volume (MEFV and PEFV) curves and the determination of Closing Volume (CV), are more suitable since they are obtained from gas flow at the mouth. A relatively inexpensive apparatus was constructed and used to measure CV, FEV 1, FVC and instantaneous flowrates on MEFV and PEFV curves produced by healthy volunteers (asymptomatic smokers and non smokers), whilst breathing air or a mixture of 80% helium and 20% oxygen (He-02). The position of a characteristic flow volume curve discontinuity (notch) was analysed in 28 volunteer (11 smokers) and was used to effect a measure of separation between the smokers and non smokers. From the systematic difference in MEF 50 and MEF 25 obtained on the MEFV and PEFV curves of 10 males (3 smokers) it is tentatively suggested that the wide predicted normal range for these indices may be related to changes in airway smooth muscle tone. An index of radionuclide particle penetration (Initial Lateral Penetration Index) was as expected positively correlated with PEPR (r= 0.52), FEV1 (r= 0.57) and MEF 50 (r=0.44) and negatively correlated with CV (r= -0.49) in 11 adults confirming that depth of deposition of particles is a sensitive function of airway health. A total of 13 indices were ranked according to the sensitivity and specificity with which drug induced reversible airway changes were detected in a group of 25 volunteers (14 non smokers). Partial (P) indices (obtained from submaximal inspiration) were ranked :- (1) MEF 25 (P) (Air) and MEF 40 (P) (Air). (2) MEF 25 (P) (He); MEF 40 (P) (He) and Isoflow volume point (IFVP). (3) The excess flow rate on He-02 compared to Air {MEF 25 (P) (He/A) and MEF 40 (P) (He/A)}. (4) CV. Correspondingly the ratings for maximal (M) indices were:- (1) MEF 40 (He). (2) MEF 25 (He), ,(3) MEF 40 (Air). (4) IFVP and CV. (5) MEF 25 (Air). (6) FEV1. (7) FVC. (8) CV/SVC (%), MEF 40 (He/A) and MEF 25 (He/A). (9) FEV1/FVC (%). (10) Slow vital capacity. MEF 40 on both air and He-02 was superior to CV and both of these were superior to FEV1 in detecting reversible airway changes.