| Summary: | We investigated the effect of rapid weight loss (RWL) and weight regain (WR) on thoracic gas volume (V<sub>TG</sub>) and body composition assessment using air displacement plethysmography (ADP) in male wrestlers. Eight male elite collegiate wrestlers completed a RWL regimen (6% of body mass) over a 53-h period, which was followed by a 13-h WR period. ADP was used at three time points (baseline (T1), post-RWL (T2) and post-WR (T3)) according to the manufacturer’s testing recommendations. The total body water and bone mineral content were estimated using the stable isotope dilution method and dual energy X-ray absorptiometry, respectively, at the same time points. Body composition was assessed with two-component (2C) or four-component (4C) models using either the measured V<sub>TG</sub> (mV<sub>TG</sub>) or predicted V<sub>TG</sub> (pV<sub>TG</sub>). Measured V<sub>TG</sub> increased from T1 to T2 (0.36 ± 0.31 L, <i>p</i> < 0.05) and decreased from T2 to T3 (−0.29 ± 0.15 L, <i>p</i> < 0.01). However, the changes in fat mass and fat free mass, which were calculated by both 2C and 4C models, were not significantly different when compared between calculations using mV<sub>TG</sub> and those using pV<sub>TG</sub>. Our results indicate that V<sub>TG</sub> significantly changes during RWL and WR, but both measured and predicted V<sub>TG</sub> can be used to assess changes in body composition during RWL and WR.
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