In the present investigation we wanted to study the functional and structural development of the acinus, using a vital capacity (VC) single-breath tracer gas washout test, modified so that the subject inspired helium (He) and sulfurhexafluoride (SF6) in oxygen. We used as indices of ventilation the slopes of N2 (SN2), He (SHe), and SF6 (SSF6) as well as (SSF6-SHe), which may be considered an index of acinar mixing linked to acinar structure. We studied two populations: Population I-49 children and adolescents (8-18 years of age); and Population II-18 non-smoking adults (22-48 years of age). Age and vital capacity (VC) were correlated in Population I, but not in Population II. In both populations there was a significant decrease of SN2, SSF6, and SHe as a function of VC. Growth and VC changes were determinants of these decreases in Population I, while only changes in VC were significant determinants in Population II. For each gas, the regression lines of slope versus VC for Populations I and II were not significantly different; we therefore suggest that the decrease in slope in Population I is mainly due to an increase in lung volume during growth. (SSF6-SHe) was not correlated with VC in Population I and was negatively correlated with VC in Population II (P < 0.01). This discrepancy implies that a child will have a smaller (SSF6-SHe) than an adult at comparable VC. Since the magnitude of (SSF6-SHe) values are related to acinar branching asymmetry, our results suggest that asymmetry of the acinus increases with lung growth and maturation.