Objectives were to describe the relations between fetal oxygen consumption (VO2), vital organ weights and body weight in sheep during growth between mid gestation and term (about 147 d). Umbilical VO2 in conscious, single-pregnant ewes and fetal wet and dry body weights were measured at 73-97 d (n = 14) and at 119-141 d (n = 28) of gestation. Fetal wet and dry organ weights were related to body weights in an additional seven single-pregnant and eight twin-pregnant ewes at 73-140 d. Fetal VO2/kg wet weight decreased by 25% between mid and late gestation, whereas VO2/kg dry weight decreased by 56% and was paralleled by a similar decline in the relative aggregate weight of the vital organs (liver, kidneys, heart, brain). Log-log regression of VO2 on dry body weight, and of dry vital organ weight on dry body weight yielded coefficients of 0.73 +/- 0.02 and 0.66 +/- 0.01, respectively, suggesting that a decline in the relative growth of metabolically active organs explains much of the decline in weight-specific VO2 during fetal development.