A method is described for continuously measuring the PO2 of lymph in conscious sheep. An indwelling polarographic electrode inserted into a plastic cannula has been used to measure the PO2 in efferent popliteal lymph close to the point at which the lymph leaves the node. The mean PO2 of popliteal lymph in normal sheep was found to be 46.6 +/- 5.3 mm Hg, equivalent to 7.1 +/- 0.8% O2. Following antigenic challenge with killed Salmonella bacteria, the PO2 of the lymph changed significantly. These changes were related to the cellular response in the lymph and in the node. The PO2 values were maximum in the lymph 8 h after the injection of antigen during the 'shut-down' period and minimum at around 80 h when the blast cell response was at its height. These in vivo findings suggest that in vitro studies on sheep lymphocytes should be done with gas mixtures containing 7% O2.