A differential body plethysmograph was developed for direct determination of specific airway conductance (SGaw) in intact conscious guinea pigs, breathing spontaneously at room temperature. Box pressure changes were measured as differences between chest volume (delta V1) and air volume respired at atmospheric pressure (delta V2) and fed to X-axis of an X-Y recorder. Airflow signal was fed to its Y-axis. A loop was formed. The slope of the rising limb of this loop provided the ratio of airflow change during transition from expiration to inspiration (delta V2) and corresponding delta (V1-V2), which is practically free from "temperature-humidity artifact" because of minimal lung volume change over this period. This ratio delta V2/delta (V1-V2), when divided by barometric pressure after subtracting water vapour pressure at body temperature (PB-PH2O), gave the value of SGaw at normal end expiration, i.e., at functional residual capacity (FRC). In 42 male guinea pigs, used in this study, SGaw was found to be 0.48 +/- 0.06 sec-1 (cm H2O)-1, comparable to the value obtained by using available data regarding lung resistance and FRC. It was independent of body weight (250-750 g). Histamine induced fall in SGaw was determined by comparing the slopes of the rising limbs of the X-Y loops obtained after and before exposure to histamine aerosol. Log-normal plots of histamine base (W/V%) and SGaw, as % of control, were used to assess airway reactivity.