Excess LW appears to be a significant factor in some neonatal respiratory disorders, and improved methods for measurement of LW have both investigative and clinical importance. Use of a double-indicator (thermal, indocyanine dye) dilution technique with microprocessor technology (Edwards LW Computer) has been extensive in adult animals and humans; however, the large injection volume(V), 10ml, has precluded use in neonates. To determine adaptability of this technique to use in neonates, LW and CO were measured in 6 lambs between 3 days and 9.5 wks of age. Animals were studied daily for 5-6 days; studies consisted of 6 injections of each of 3 volumes (2,5, and 10ml). Mean coefficients of variation for LW for each V were 11.6 to 15.5%; those for CO were 6.5 to 13.4%. Correlations between V and log LW were -0.81 to -0.96 (p<0.001); between V and log CO they were -0.92 to -0.97 (p<0.001). Ratios(R) for the 10:5ml V and the 10:2ml V, respectively, were: LW=0.421±0.048 ([Xmacr ;[plusmn;SD) and 0.115±0.024; C0=0.480±0.015 and 0.158±0.010. Gravimetric measurements (GM) of LW were 4.36 to 5.52ml/g dry wt and 16.2 to 19.1ml/kg body wt (N=3). Mean values for LW estimated using R from 10:5ml and 10:2ml differed from GM by 12.4 and 10.7%, respectively. CO was similar to previous reports, 230 to 535ml/kg min. These data are suggestive that the double-indicator dilution technique can be modified for the use of a smaller V (2 or 5ml), and thus may have potential application in the neonate.