The distribution of inspired gas to each lung, time constants of the lungs and parameters of gas exchange were studied in 2 groups of horses (mean bwt 606 kg), anaesthetised using thiopentone and chloral hydrate and breathing room air. One group (n=4) had a downward curved abdominal contour (round-bellied) and the other group (n=4) had an upward curved abdominal contour (flat-bellied). An equal distribution of inspired gas between the lungs existed in both groups in dorsal recumbency. Flat-bellied horses maintained this equal distribution in lateral recumbency whereas in round-bellied horses an uneven distribution of tidal volume (VT) developed. The percentage of (VT) distributed to the dependent lung was 23% and 38% for left and right lateral recumbency respectively. The distribution of VT agreed with the ratio of time constants of the lungs in flat-bellied horses but differed markedly from this ratio in round-bellied horses suggesting that, in the latter, factors other than compliance and resistance play a role in distribution of ventilation. Round-bellied horses had a lower PaO2 and a larger (A-a)PaO2 than flat-bellied horses in all body positions. The results are compatible with the known hypothesis that pressure exerted by abdominal contents on the dependent lung and diaphragm is an important factor in ventilation/perfusion mismatch of the anaesthetised horse.