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1. Experiments were done on exteriorized foetal lambs of 123-144 days gestation to measure bidirectional ion fluxes through the pulmonary epithelium and to compare them with those predicted from the sum of the measured forces determining passive flux according to the Ussing flux-ratio equation. Fluxes of Li+, Na+, K+, Rb+, Cs+, Mg2+, Ca2+, Sr2+, Ba2+, Cl-, Br- and I- were measured and permeability constants obtained by following concentrations of their labelled isotopes in lung liquid and plasma after injection into either. Activity ratios were obtained from chemical measurements or tracer distributions, electrical potential differences by placing KCl-agar bridges, connected to calomel half-cells, in blood and lung liquid. Lung liquid volume and secretion rate were measured by adding an impermeant tracer (inulin or [125I]albumin) to lung liquid.
2. The permeability sequence of the pulmonary epithelium for alkali metals was, Na+ > K+ > Rb+ > Li+ > Cs+ and that for halides I- 
Br- > Cl-. Permeabilities to alkaline earths were lower than for the other ions, no definite sequence being established.
3. There was an electrical potential difference of -1 to -10 mV (mean -4·3 mV) between lung liquid and plasma (lung liquid negative). Plasma/lung liquid chemical activity ratios were less than unity for the halides (Cl-, Br-, I-), and for K+ and Rb+, whereas the ratio of one-way fluxes (plasma 
lung liquid)/(lung liquid plasma) was in each case greater than unity. From the difference between the measured flux ratios and those predicted from the forces determining passive flux, it was concluded that the halides, K+ and Rb+ were actively transported from plasma to lung liquid, Cl- being quantitatively the most important. Na+ and Ca2+ appeared to move passively down a gradient of electrochemical potential.
4. When alveolar liquid [HCO3-] was artificially raised, a net flux of HCO3- from lung liquid against a gradient of electrochemical activity was observed, suggesting active transport of that ion out of lung liquid.
5. The addition of KCN to lung liquid stopped the secretion of liquid and absorption took place.
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