The present study was undertaken to devise an accurate method for measuring functional residual capacity, FRC, in small and lively newborn animals such as lambs and piglets under flexible laboratory conditions using relatively simple equipment. This was achieved by adapting the standard helium dilution method so that FRC could be calculated from the helium equilibration concentration reached when the subject, having breathed a gas mixture with a high helium concentration, rebreathed from an air-filled spirometer. Using this technique, typical results for test volumes of 10, 20, 30 and 40 ml were measured as 10.7 +/- 0.34, 20.2 +/- 0.19, 30.5 +/- 0.40 and 40.6 +/- 0.75 ml respectively with corresponding coefficients of variation of 6.4%, 1.9%, 2.6% and 3.7%. Repeat measurements made on each of 3 urethane anaesthetised rabbits gave values for FRC of 45.64 +/- 0.9 ml (5 measurements), 37.22 +/- 0.8 ml (8 measurements) and 48.95 +/- 0.99 ml (8 measurements) confirming the reproducibility of results from animal preparations. The FRC of newborn lambs (12 animals) was 81.4 +/- 9.2 ml. The technique also allowed clear recordings of respiratory pattern essential to the analysis of variations in FRC.