1. We studied a total of 29 patients on continuous ambulatory peritoneal dialysis (CAPD), who had no present or past respiratory impairment, before and after drainage of dialysate (2.324 ± se 0.033 litres).
2. We measured spirometry, lung volumes and carbon monoxide transfer sitting and supine in 20 patients. The only statistically significant changes on drainage were a small increase in supine functional residual capacity (+ 214 ± se 61 ml, P<0.01) and a small fall in supine peak expiratory flow rate (−26.6 ± se 12.1 litres/min, P<0.05).
3. Measurement of maximal mouth and transdiaphragmatic pressures in ten patients made under the same circumstances showed no statistically significant changes on drainage.
4. A model of the abdomen demonstrates that fluid distension is likely to be better tolerated than gaseous distension, and review of previous studies suggests that a wide range of changes in intraabdominal fluid volume can be tolerated without respiratory embarrassment.
5. These results suggest that fluid volumes used for CAPD do not interfere with breathing in patients with previously normal lungs. The nature of the mechanisms compensating for the fluid suggests that patients with chest disease should be able to tolerate this form of dialysis.