Effects of recumbency on nasal patency were determined in adult subjects by a sensitive computer-assisted posterior rhinometric technique. A modified body plethysmograph was employed to measure air flow and to avoid the disadvantages of facial masking and nasal intubation. In healthy subjects, assumption of recumbency decreases total nasal patency minimally. Lateral recumbency decreases it markedly in the ipsilateral nasal cavity, in which resistance to breathing often exceeds 10 cm H2O (1.0 kPa)/L/s. A concomitant increase in patency, which takes place contralaterally (resistance is frequently reduced to less than 3 cm H2O (0.3 kPa)/L/s), diminishes change in total nasal patency. Ipsilateral decrease and reciprocal contralateral increase in patency are induced also by unilateral pressures to trunk and limbs in dorsally recumbent subjects. This posture minimizes hydrostatic differences between sides. The results indicate that reciprocal nasal vascular responses to lateral recumbent postures are caused by pressure-induced reflexes, and these changes take place independent of hydrostatic differences. Effects of recumbency on the patency of abnormal noses and their relevance to breathing disorders of sleep are discussed.