Rapid eye movements during rapid-eye-movement (REM) sleep are associated with rapid, shallow breathing. We wanted to know whether this effect persisted during increased respiratory drive by CO₂. In eight healthy subjects, we recorded electroencephalographic, electrooculographic, and electromyographic signals, ventilation, and end-tidal PCO₂ during the night. Inspiratory PCO₂ was changed to increase end-tidal PCO₂ by 3 and 6 Torr. During normocapnia, rapid eye movements were associated with a decrease in total breath time by 0.71 ± 0.19 (SE) s (P < 0.05) because of shortened expiratory time (0.52 ± 0.08 s, P < 0.001) and with a reduced tidal volume (89 ± 27 ml, P < 0.05) because of decreased rib cage contribution (75 ± 18 ml, P < 0.05). Abdominal (11 ± 16 ml, P = 0.52) and minute ventilation (0.09 ± 0.21 ml/min, P = 0.66) did not change. In hypercapnia, however, rapid eye movements were associated with a further shortening of total breath time. Abdominal breathing was also inhibited (79 ± 23 ml, P < 0.05), leading to a stronger inhibition of tidal volume and minute ventilation (1.84 ± 0.54 l/min, P < 0.05). We conclude that REM-associated respiratory changes are even more pronounced during hypercapnia because of additional inhibition of abdominal breathing. This may contribute to the reduction of the hypercapnic ventilatory response during REM sleep.