We studied the ratio of the expired He and SF6 concentrations (He/SF6) after 1 litre inspirations of a gas mixture containing 5% of He and SF6. Five subjects aged 49 to 60 yrs. performed the maneuvers with both inspiration and expiration at 0.2--0.41/s or 1.5--2.01/s. In all subjects separation of the tracer gases was observed, the He/SF6 falling early to a minimum of 0.80 +/- 0.01 (mean +/- 1 SE), and increasing gradually during expiration to a maximum of 1.08 +/- 0.01. The slope of the SF6 alveolar plateau was 1.45 +/- 0.09 times that for He. Computer simulations of simultaneous convection and diffusion in an axisymmetrical series lung model predicted a pattern of He/SF6 early in expiration which corresponded qualitatively to that observed experimentally. However, the model did not predict a rising He/SF6 ratio late in expiration. This was simulated only by incorporation of parallel inhomogeneity with sequential emptying into the model analysis. Consideration of Taylor type dispersion and airway asymmetry did not influence the simulations significantly. The results suggest that differing slopes of the alveolar plateau of two tracer gases may be due to diffusion dependent concentration differences among lung units ventilated in parallel rather than due to stratification alone.