Previous studies during head-out immersion have shown closing volume (CV) to either increase or remain unchanged. We hypothesized that these inconclusive results might be related to differences in the ages of the subjects tested. To elucidate this we studied single-breath argon washout tests performed by a younger group (n = 8, age 23-26) and an older group (n = 8, age 40-54) of males every 5 min during 30 min of seated, thermoneutral head-out immersion. No temporal changes in CV during immersion were observed in either group, therefore values within each group during immersion were combined. In the younger group, CV increased 77% (dry, 0.26 +/- 0.11 L; wet, 0.46 +/- 0.10 L; delta = 0.20 L) (P < 0.001) but remained less than the tidal breathing range upper limit [expiratory reserve volume + tidal volume (ERV + VT)]. In the older group, CV increased 34% (dry, 0.83 +/- 0.29 L; wet, 1.11 +/- 0.19 L; delta = 0.28 L) (P < 0.05) and was not different from ERV + VT. The absolute increase in CV during immersion did not differ between the groups. ERV decreased during immersion in both groups and was lower in the older than younger group (P < 0.001). Alveolar plateau (phase III) slope became steeper in the younger (P < 0.001) but not in the older group. We conclude that during immersion: (1) The absolute increase in CV is independent of age, and (2) in subjects over 40, CV approaches the highest lung volumes reached during tidal breathing.(ABSTRACT TRUNCATED AT 250 WORDS)