A study was conducted to clarify the influence of water immersion at different levels on pulmonary response. The subjects, ten healthy men (mean age, 26.2±7.9 years), subjected to measurement of static lung volumes and maximum expiratory flow-volume curves while sitting immersed in water at the level of both the neck and diaphragm. TLC, VC, ERV and FRC for water immersion at the diaphragm level were significantly decreased in comparison with those measured in air. These lung volumes were further decreased upon water immersion to neck level. However, RV did not change significantly upon immersion at either water level. Significant decreases of FEV_1⋅0, FEV_1⋅0%, V₅₀ and V₂₅ were observed upon water immersion at the diaphragm level as compared with those obtained in air. Water immersion to neck level produced further decreases in pulmonary functional parameters. Although peak flow and V₇₅ did not change significantly upon water immersion at either level, V₅₀ and V₂₅ were decreased markedly in comparison with the values obtained in air. A tendency for a marked decrease in pulmonary function parameters was observed upon water immersion to neck level. The changes observed upon water immersion to diaphragm level may have resulted from compression of small airways induced by both an increase of blood volume in the thorax and hydrostatic pressure against the abdomen. The changes induced by water immersion to neck level may have been exacerbated by the two mechanisms described above, in addition to hydrostatic pressure on the chest wall. The present results suggest that the significant reduction of pulmonary functional parameters caused by water immersion may be due to compression of small airways induced by an increase of blood volume in the thorax and hydrostatic pressure on the chest wall and abdomen.