We tested the hypothesis that isovolemic anemia blunts the thermogenic response to environmental cold stress in 3 to 4-day-old newborn piglets. Eight animals were studied in both thermoneutral (31.6-32.8° C) and cold (19.6-20.2° C) environments, before and after an isovolemic, partial volume exchange transfusion which reduced the hematocrit from 26 to 15 %. In the non anemic phase of study, deep rectal temperatures declined but had plateaued by 30 minutes after onset of cold stress and remained within normal limits for newborn piglets. In the anemic phase of study, deep rectal temperature declined continuously throughout cold stress with true body core hypothermia (<38° C) observed at all measurement points beyond 15 min of cold stress. Baseline oxygen consumption did not differ between the two study phases (17.6 ± 1. 8 versus 16.7 ± 2.1 ml/kg−1min−1, mean ± SEM). However, during environmental cold stress, oxygen consumption increased by 64% over baseline in the nonanemic phase of study (p<0.05) whereas 21% increase over baseline was observed in the anemic phase (p NS). We conclude that isovolemic anemia limited oxygen consumption and heat production during environmental cold stress, resulting in body core hypothermia. (Pediatr Res21: 482-486,1987