Simultaneous washouts of nitrogen (N2) and helium (He) boluses were performed in 9 seated symptom-free asthmatic patients. Recent work has indicated a correlation between the height of Phase IV (deltaIV) of a vital capacity washout and the end-inspiratory vertical concentration gradient of the gas in the lung. The deltaIV for both He and N2 washouts increased by a factor of 2 and 1.25, respectively, after isoproterenol inhalation. In addition, the amplitude of the cardiogenic oscillations increased from a mean of 0.32 per cent to 0.51 per cent for N2 and from 0.030 per cent to 0.051 per cent for He. Closing volume measured by the He bolus method was higher than that by the N2 method in 8 of 9 subjects, by 3.7 +/- 0.7 per cent vital capacity (mean +/- 1 SE, P less than 0.001). Similarly, He closing capacity was greater than N2 closing capacity in the same 8 subjects by 2.6 +/- 1.3 per cent total lung capacity (P less than 0.001). Despite a decrease in closing volume in 8 subjects, and in closing capacity in all 9 subjects after isoproterenol, the differences between the results obtained by the 2 methods remained unchanged. The slope of Phase III in both N2 and He washouts was influenced in an inconstant fashion, probably reflecting differing contributions from topographic and intraregional inhomogeneities of ventilation in these subjects. The results suggest that in patients with asymptomatic asthma the vertical gradient of regional residual volume as well as the pattern of airway closure is more uniformly distributed between apex and base of the lung. Furthermore, the slope of Phase III in either N2 or He bolus washouts may not reflect the degree of bronchoconstriction.