The sniff maneuver has recently been advocated for assessing inspiratory muscle strength. We characterized the relationship between mouth pressure (Pmo) and esophageal pressure (Pes) during sniffs performed with open, semi-occluded, and occluded nose. In seven normal subjects, pressure was simultaneously measured with a mouthpiece (Pmo) an esophageal balloon (Pes) during high- and low-intensity sniffs performed from FRC. With open nose, the mean ratio Pmo/Pes was 0.96 +/- 0.05 (mean +/- SE). The mean ratio Pmo/Pes was not significantly different in high- and low-intensity sniffs. With one nostril occluded, the mean ratio Pmo/Pes was 1.03 +/- 0.04. The ratio Pmo/Pes was significantly higher than with open nose for all sniffs (P less than 0.05) and low-intensity sniffs (P less than 0.01). With both nostrils occluded, the mean ratio Pmo/Pes was 1.06 +/- 0.03, with occasional marked overestimation of Pes by Pmo. We conclude that Pmo generally reflects Pes during sniffs performed with open and semi-occluded nose, but not with complete nasal occlusion.