Based on earlier results on a body size relationship of blood heat output in different mammalian species, an intra-specific comparison of blood heat output in human neonates and adults was made. To avoid taking additional blood samples from neonates, a resuspension technique was developed, allowing the use of material left over from clinical routine sampling. In 13 adult volunteers, microcalorimetric heat output ((x) over bar +/- SEM) was 48.1 +/- 0.8 mu W (ml(blood))(-1) or 105.6 +/- 2.2 mu W (ml(cells))(-1) in the whole blood and 47.3 +/- 1.6 mu W (ml(susp).)(-1) or 104.0 +/- 3.3 mu W (ml(cells))(-1) in the resuspended samples, respectively, proving that the resuspension procedure did not alter the thermal power values. Using this technique on blood samples from 38 neonates in their first week of life, a heat output of 77.2 +/- 3.1 mu W (ml(susp.))(-1) or 151.8 +/- 5.9 mu W (ml(cells))(-1) was found. This is significantly higher than in adults, and fits the body size relationship of blood heat output previously described for different mammalian species.